Thanks for the reply leafo.
Yes, I'm using umatrix in the browser and privoxy in the router, but I'd already tried bypassing both those.
I'm not really an 'app' sort of chap, so in the end I've installed a different browser on a USB stick for use with itch, and games are running OK on that. It's like using a dedicated app, but with more buttons & switches exposed. You can never have too many buttons and switches!
Cheers,
Tim