Please note that, in typical fashion for me, I think I was overcomplicating the issue when I set about my testing

After the testing was complete, AND, after I had typed all that out, I looked at the results again and realised a simple (suitable) potentiometer (in the +ve line) would likely work fine. I just did not have a suitable one to hand., So, I added the E D I T, as per =
ghost123uk wrote: E D I T = reading back over my results below, if I had a 15W 500Ω pot, I would have tried itLooking again at the results, i think it might be OK.
So, for the price of one, get a simple 15W (12W would very likely be fine, and of course bigger will do no harm and won't alter the results, so say a 20W one would do fine) wirewound 500Ω potentiometer. If you can't find a 500Ω one, 680Ω would also work. I'm sure it will do what you need.