I reckon those Btwin Rockrider mountain bikes are on par with the Boardmans for the same money. They are very nice in the flesh (like the Boardmans) and they have some good reviews!
IIRC The Btwin came out top on a Gadget show test too.
From what I've seen in Decathlon and Halfords I'd be different and get the Rockrider.
Have u thought about spending £500 on a 2nd hand bike?
You could get a beaut