Google "Snugpack" - their factory is in Silsden, 'just around the corner' from DT. Look at their "Softie" range. I have the lightest model in that range the "Softie 3 Merlin" which weighs approx 1000g. The professional Alpine guides in Cham reckon to use them for bivvying down to -5*C. A mate of mine has one that he uses every year for the OMM ( late October ) and he's usually in the top dozen at most Lakeland AL's - no slouch!
That range goes up to the "Softie 12 Osprey", about 1800g. Then again, her indoors has a "Softie ? Ultimate ?" which looks like it should be on an Antartic expedition. We had to wait about 6 months for it, as Snugpack's production of this model was tied-up with an order for 6,000 from the Latvian army!!