http://www.chainreactioncycles.com/sunn-tzar-s1-hardtail-bike-2013/rp-prod121793
Not a 29er but perfect for trail centres , and a bargain !

Bought my son a Scott 29 from Evans in the sale for slightly more but <£500

http://www.paulscycles.co.uk/m1b0s155p0/Mountain-Bikes/Front-Suspension-29ers

Usually good deals to be had here. Limited sizes though

No experience of this, but looks like a bargain

http://www.chainreactioncycles.com/vitus-bikes-nucleus-290-hardtail-bike-2014/rp-prod107044

Sign up for the halford emails, they do regular 10% off for a couple of hours every couple of weeks, that and the 10% off instore with a British cycling and more via quidco, I’d be looking at the extremely well rated Voodoo Bizango.
Ok so it maybe a little more than £450, but it’ll be a better bike for it.

That said, lot of these Bizango’s ebay, and if you ignore the chancers trying for £450+, there’s regularly now bikes coming up around £350-£400, which are probably have little or no use (bike to work scheme purchases).