I got a Ragley Piglet from eBay for £400….it had a crappy 120mm Recon fork but i had a 140mm Sektor at home so the bike has turned out great for not a lot of money.
Trawl eBay, some complete On-One 456 Summer Season builds come up surprisingly regularly for about your price range….or get a 2nd hand fork from these guys:
http://flatoutcycles.co.uk/
….their stock changes daily but i’ve seen decent forks on there for less than £200, they currently have some 140mm Sektors for £140 and some Fox 32 150mm for £200.
Then get a Dartmoor Hornet from here:
http://slam69.co.uk/
….the frame is £185 brand new.
I reckon for less than £350 you could have a new frame running on good second hand forks, you can pick up wheels for nothing on eBay or On-One do their Reet’ard set for £100 and they’re fine….been running them on a hardtail for over a year now with no problems.