This one…
http://www.chainreactioncycles.com/vitus-bikes-sentier-290-hardtail-bike-2014/rp-prod107048

Or this one
http://www.halfords.com/cycling/bikes/mountain-bikes/voodoo-bizango-29er-mountain-bike-20

or any other ideas around the £600 bracket

I would go with the voodoo with the better fork, the only way to something “better” would be 2nd hand.

The Voodoo Bizango a very well rated cheap 29er.
As long as your in no major hurry wait sign up for the halford email and buy on a discount day, British Cycling members have 10% off with them instore. Quidco does 4.5% cashback, or just search round the ‘net for Halford discount codes (can’t use BC discount conjunctive though). There also talk you can buy Halfords vouchers cheap and use them to pay…
At worst, they sell (well are advertised) for about the price of the discounted version on ebay, if you end up hating it 😀