Well, I've solved a little of the mystery: the tiny weeny bugs I found in Percy's house are indeed Demodex mites (took two to be professionally ID'ed under microscope). So, the normal mites which appear on a hamster and live on dander and skin flakes. They are far tinier than a pin head but you can see them move if you have them on a light background and see them in your peripheral vision. I wear glasses and don't have that great eyesight but it's interesting that contrary to the usual accepted idea that you can't see hamster mites at all, you obviously sometime can - especially with a bright light (I use my mobile's torch) on them. Since I first wrote I've found about 4 or 5 of them on his wooden house lid but no more. You can squish them with a hard press of the finger and they just disappear if you do.
Demodex are apparently arachnids (yup - mini spiders!!!) and usually live on harmony on a ham without proliferating unless they get stressed. My guess is when Percy was new and peeing in his bed, it got humid in there and he was a bit under the weather with the change of home from the breeder's etc, and a few of them bred more. I've not found any since that and he's not scratching any more than an average washing-obsessed ham!
So... not going to worry too much but thank you all for your insights as I was definitely freaking out!! And, alongside it all Percy has taken to taming beautifully and is now so completely okay about scrambling all over me it's hard to remember a month ago when he was the most terrified little ham! He's happy and so I think any mites have been put back in their place - though I will always monitor to ensure there aren't further population explosions. Percy's house is for one critter only: Perseus!
