A 17" frame is gonna be far too big to ride trials on, years ago when you could only really adapt a mountain bike I was riding a 13" Specialized Rockhopper and when I look back at pics of that now it looks huge! I definately agree with the comment about the MAD Phase 1.2, for £250 it's a great bike to start with, decent enough geo (the team riders use the same frame, look for some vids on their website) and is a good enough bike to look at upgrading. If you end up buying a £200 2nd hander off ebay and decide you really wanna give trials a go and progress your most likely gonna have to go out and buy everything all over again to build a "proper" rig whereas with the 1.2 it's not much over your budget, perfect for learning on and suits high level riders with a few choice upgrades.