To be perfectly honest, I think it would be a good idea to try and locate some riders in your locality, and if they'll be so kind, have a go on their bikes, so you can get the feel for different styles and types of trials bike; Otherwise you could buy a trials bike, not like it at all and then lose money and time selling it afterwards. For £500-600, the T-Rex is a brilliant option, i'm pretty sure it's common opinion that the T-Rex is a really nice bike for beginners or pros alike.
Edit: Crap, just re-read all of that and realised you've done trials before (Y)
In that case, get a T-Sex anyway, or i'll just shutup, you just ignore my post and I go to bed... (Y)