just istalled froyo 2.2 "I9000XXJP3 version" using odin and also RyanZAEXT2LagFix ALPHA 1. my question is this - can i revert back to 2.1 ? if so could somebody please explain how in some simple steps or point me in the right direction... im not bothered about loosing stuff... any help would be great, thanks in advance

Just download a 2.1 rom and reflash via odin. Since you have done it once already, you should know the trick Reflashing will remove root.

I've done something similar except I didnt check what the original firmware was before I did it. Whats a good basic one to use (for o2 in the uk)

Check samfirmware. I am not sure what the country code for UK is. You will have to find that out somewhere.