I wouldn't worry about the 3 yrs thing, I'd happily ride a 15 year old MTB off road. If you've not ridden a decent MTB before, a 6 year old bike would be a revelation, and you might not fully appreciate the subtle improvements of more modern bikes if you're mainly ploughing into trees.
I'd be more concerned about it not needing any expensive work on any hydraulics. At £500 avoid full suspension - extra weight, extra complexity, extra cost.
I wouldn't worry about the 3 yrs thing, I'd happily ride a 15 year old MTB off road. If you've not ridden a decent MTB before, a 6 year old bike would be a revelation, and you might not fully appreciate the subtle improvements of more modern bikes if you're mainly ploughing into trees.
I'd be more concerned about it not needing any expensive work on any hydraulics. At £500 avoid full suspension - extra weight, extra complexity, extra cost.
Although, that said, if you could stretch to £550: http://www.ebay.co.uk/itm/2009-marin-wolf-ridge-/170750645985?pt=UK_Bikes_GL&hash=item27c18816e1#ht_500wt_1202
More realistically, you can get something very decent for a few hundred: http://www.ebay.co.uk/itm/Kona-Kula-/220914745817?pt=UK_Bikes_GL&hash=item336f8b81d9#ht_1242wt_1185
http://www.ebay.co.uk/itm/Specialized-stumpjumper-m4-chrome-moutain-bike-/260919487721?pt=UK_Bikes_GL&hash=item3cc0036ce9#ht_500wt_1202