In reply to humptydumpty:
> If a £400 bike is unrideable for you, how much is a mid-range bike?
2k for my full suss, 1200 for my hardtail. I could have spent more, but it's a question of diminishing returns and the fact that wouldn't appreciate the difference.
Forks are a good case in point - you can pay anything from £100 to £1000. The more you pay, you get a smoother ride and more features, but those features are only worth it you put in the effort to set it up right and adjust according to the trail you are riding. My problem is that I want a bike I can set up, leave it and forget about. And whilst I can lockout both the fork and rear shock - better for climbing - if I do I invariably forget to reset when I start the descent, and may not realise til I'm down (and wondering why the bike is bouncing all over the place). On the other hand I have a very cheap fork on an old bike (was going to be a spare MTB, now a commute bike) and when I do bounce down some steps I can really feel it.
All depends what you are likely to do. Obviously you don't want to spend 2k if you aren't sure you'll get into MTBing, but you might find someone else whose made that mistake and has a decent bike to sell. Or spend £600-700 now, ride it for a few years and even if you decide you want something better you'll still have a useful spare or commute bike (the n+1 rule applies more for MTBs than road bikes - they are more likely to need work).
Demo days are a really good way of trying out bikes, have a look round for any near you. Or go to a trail centre and hire one,
I guess my point is that a good bike will be fun to ride and encourage you to get out riding, if you don't enjoy it, the bike will be neglected and a waste of money. It was pissing down yesterday, but I still did an extra lap of Kirroughtree because I was having so much fun !