RE: Okay, therefore is the current at a fixed and constant rate in your device
Yes.

RE: Do you know what current flows between the electrodes?
In my own case that would be 32 mAh

RE: Ionic silver by comparison is largely ineffective as it reacts with chlorides within the body, in particular the stomach which would render it pretty much useless
I too recall feeling this way in the early stages of my research. That said, upon closer inspection, I discovered that these particular views stemmed from studies and reports carried out by certain Colloidal Silver manufacturers who were intent on promoting their products. Whereas in truth, the published report data would not prove substantial to the claims in that the numbers are reflective of the sample concentrations. Ie, 20ppm of Microparticle Silver vs a 10ppm Ionic Solution. 

That said, I’d encourage you to download and review this report as well as the following tests to draw your own conclusions toward matters.

In saying this, it has been my observation that all forms of Silver Solutions(compound or particulate) will undergo disassociation in the presence of HCl, whereas specialty products such as encapsulated solutions have been reported to survive gastric conditions in excess of 60 mins. 

RE: As I understand it Ions are more effective for topical use where chlorides are not present but inside the body there are a lot of chlorides which make them far less effective.
I encourage people to conduct their own tests in cases such as these, as this will likely prove far more beneficial in terms of conclusions than to take someone to their word. That said, one of the biggest issues with the Colloidal Silver domain in our day, is where it is subject to vast amounts of personal opinions and repetition. Which often leads to what I’d call personal opinions eventually taking-on the form of verfified facts given enough time. - unfortunately

RE: What made you decide on 42V DC?
42v marked the necessary voltage required to ensure a current flow of 32 mAh was supplied to the cell from start to finish.

That said, I’d remind you that the voltage in this particular case is merely a carrier through which the current is delivered into the cell. And so in this way, one could conclude that the voltage is secondary to the objective so long as there is sufficient voltage to successfully carry out the flow of electrons. 

NB. One simple way to visualize this would be to assemble a simple Electrolysis circuit along with an active measure of the voltage reading across the electrodes(anode and cathode), as well as a means from which to monitor or measure the current flowing through the cell. After which, activating this(w/distilled water) will likely show a full voltage reading(ie, 30v) and very little current. Which will in turn, increase as the leve of ions in the concentration begin to rises.
 
That said, what's interesting with this, is where we can influence /or change the level of current going into the cell at this stage using several methods:

1. Alter the physical spacing between the Anode and Cathode(bringing them closer together)
2. Increase the surface area of the Anode and Cathode so as to improve conductivity
3. Heat the water so as to increase its thermal conductivity
4. Raise the voltage(pressure) so as to increase the current potential in the cell.

And thus, it becomes possible to control the rate of current flowing into the cell by adjusting the voltage in the early stages of processing or as is commonly referred to as the ramp-up stage.

What are you using to provide that 42V DC?
I'm currently using a variable bench-top power supply if not, only for convenience. That said, I've had success using a conventional power supply(from an old printer) coupled with a DC-DC step-up voltage converter and rheostat(to control voltage). As well as a using a 60v DC LED power supply, which seemed to work well also. 

RE: Smaller particles seem most effective. 
I agree. Though I'd also add diffusion potential(permeability) and reactivity (through ionic exchange), at the top of this list as these seem vital to the effectiveness of a solution in most cases. 

RE: thesilveredge.com claims that their device can make particles as small as 0.8nm which is fairly small, not quite as small as products professionally made by MesoSilver but still very close if what they say is true. Do you think that their claims are possible? 
Theoretically, the silver ions liberated from the anode through electrolysis will prove to be the smallest physical particle size possible this side of the material barrier. Beyond this, the issue of reduction and net structure(particle dispersion) will come into play so as to determine the overall quality of the particulate in solution. To which I'd add, I'm happy to say, science is on our side:

In short, if a seller claims to have a +20ppm Colloidal Silver solution, while claiming to have the smallest particle size only to turn and have the darkest solution, then we can rest assured that something is amok with the claim. 

That said, I recently witnessed a lab grade +30ppm Colloidal Silver solution, made from the smallest physical particle size made possible through modern science and will say with assurance that there is no such solution(to my knowledge) being sold anywhere. - I have never seen anything like it. 

Similarly, I would say with great assurance that companies or sellers claiming to make .6nm particulate solution only to end-up with a tea colored solution, should be taken with a rather large grain of salt - my two cents of course

RE: What I would like is something which works well, gets results but isn't too expensive. 
Fair enough. 

RE: Stomach acid will convert Silver ions so silver particles of small sizes are essential. 
I do hate to be the barer of bad news on this, though I'd add that the acids in our stomach's will convert any form of silver particulate, be it ionic or elemental, regardless of the composition. Which can be verified at home by taking a solution such as Mesosilver etc. and adding a few drops(0.1M) of HCl and observing the results. 

And so at best, the best outcome in such cases would be to work toward improving resistance and contact time so that the particulate can survive long enough in the digestive tract to make a difference.

Having said that, your needs will definitely pose a challenge in that most LVDC process methods will require additional measures to either stabilize the solution and /or improve resistance.  ie, encapsulation 

RE: That is why I asked the questions I have as the design I have ATM does seem a bit deficient in that respect and uses fairly high currents of around 10mA. I do not see production time as being as important as quality. I need to give it maximum chance of success.
I'd add that the initial size of a colloidal silver particle is only one part of the quality criteria. Though I'm thinking this will not be your greatest obstacle given that the colloidal silver is destined for the gastric system. To which I'd add, all potential for the structure and where the quality of the solution will become null-and-void once it enters the stomach - personal testing has shown that the disassociation time for silver nanoparticle in typical conditions(pH) is around 5 -30s at best. 

RE: Being able to adjust the current would be beneficial as I might be using it for multiple purposes both internally and externally. Versatility would be an appealing feature. 
The total current is determined by the inherent capacity of the anode(dia., and wetted surface area). And therefore should not change unless the anode size is changed, upgraded etc. 

RE: I can see a problem with this method in that polarity reversal would be very difficult if using a diode where current can only travel in one direction. I did notice another post of yours about 13 months back where there was mention by you and another poster about build up on the electrodes which indicates that you do not use polarity reversal which can combat that problem. 
I choose not to make use of a polarity reversal as this promotes the release of silver oxide in solution as opposed to aggregating on the cathodes(which can then be removed from the solution). I've also determined that this particular practice greatly affects the overall quality of the solution and stability. That said, it would take two current limiting diodes(back to back) to control current in a polarity reversing circuit.

RE: I noticed that you mention heating the distilled water in the thread. What benefit does this give? Does it affect Silver Ions, get rid of them or prevent them from forming?
Heating ones solution does a number of things, including though not limited to; raising conductivity, and electrode capacity, reducing oxides(in solution), increasing solubility(compounds), reducing oxygen and increasing hydrogen, as well as promototing the reduction(converting) of the silver ions into elemental particles. 

That said, one of the primary methods of controlling the ionic /particulate ratio of a solution is accomplished through the use of heat. 

RE: I would like something which provides polarity reversal, current adjustment so that variable particle size is possible and being able to produce the smallest particles that I can. 
Fair enough. However, I've never designed a reversing polarity circuit personally, and so I'm afraid I won't have much to contribute on this. That said, I did however, see a few videos in the past as well as a few circuits floating around that may warrant taking a look if this is your objective.

RE: ...It's a question of how low such a comparitor can go and what range they could operate to.
I'd begin by calculating your minimum and maximum current needs; ie, 1 mAh p/squ. in. of wetted Anode surface area, for the circuit and take it from there.

PS. Based on what you've written, I'd question the costs of your endeavor in contrast to purchasing an advanced CS generator personally.

- hope this helps