Friend of mine has set me a task to find a decent full sus bike for his 14 year old. He's about 5 feet 9 inches, so a medium size will be ok for now and a year or three from now.
So, what's good value around the £900 mark? 100mm to 150mm travel, but 120 is probably the sweet spot. Forest of Dean riding, plus going down 'the jumps' with his mates.