I know youre saying HT, but here's a bloody bargain.
https://www.chainreactioncycles.com/p/vitus-sommet-29-frame-2020?color=black&sizeStandard=xlVitus Sommet suss 29er inc am RS super deluxe select R. Not the best of shocks, but they cost £130 which brings the frame(also inc headset,S/clamp and rear axle) down to £200.
Also available in 27.5" at the same price(£329)
That said, they also have a Nukeproof Mega 275 Carbon suss Frame for £99 over your £400 budget. Which comes with a RockShox Super Deluxe Ultimate RCT, which would normally set you back £350 itself - and comes with the headset, S/clamp and axle. £500 for a carbon susser is pretty damn remarkable, especially as its a nukeproof and normally retails at near 5k for a full factory build.
https://www.chainreactioncycles.com...rame-2020?color=concrete-grey&sizeStandard=xl
Im desperately trying not to buy either
If you are set on an HT, they've a Ragley Blue Pig Hardtail Frame reduced from £550 to £320
https://www.chainreactioncycles.com...dtail-frame-grey-deep-sea?color=deep-sea-dive
Other bargains, HT frames, forks etc etc
https://www.chainreactioncycles.com/c/bike-parts/frames-and-forks?page=2