This is the problem with mechanicals. Rather than putting V brakes on (It could be that you have disc specific forks, although at the price bracket which uses mechanicals I shouldn't thinks so) you might consider putting a decent set of hydraulics on. A viable set of avid 3 or deore might do the trick. A replacement crank arm LHS should cost you £15. If it is RHS, then it is a bit more. Consider buying Deore Hollowtec II chainset for £60.
If those few jobs (Brakes and crank) cost as much as the bike, you need to look at upgrading, as you say, since it sounds like you have outgrown you existing.
Try this:
http://www.evanscycles.com/products/specialized/hardrock-disc-2010-mountain-bike-ec019400
The V brake equivilent is:
http://www.evanscycles.com/products/specialized/hardrock-2010-mountain-bike-ec019401
However, if you want a bike that is going to be fun to muck about in the woods, you need to spend at least £500.
These are not bad:
http://www.evanscycles.com/products/scott/aspect-20-2009-mountain-bike--ec016255
A bit heavy and kit is only OK.
http://www.evanscycles.com/products/kona/blast-2009-mountain-bike-ec017217
Mechanical brakes and 24 gears, but a well set up machine. Should give you a lot of fun. Got an MBR 10
http://www.evanscycles.com/products/scott/scale-70-2009-mountain-bike-ec016239
Race bike. Not good for trail centres, but fast for XC
http://www.evanscycles.com/products/gt/avalanche-10-disc-2009-mountain-bike-ec016364
Viable kit and nice ride, but a bit heavy.
Luck