It's not an easy question to answer, because there are a few variables. The basics are that it must be a 3 to 4 season sleeping bag. So with that as a starting point you need to figure out if you'd prefer a mummy style or a square shaped one, do you want a hood or a half crescent place to put a pillow. As for makes, the ones that get mentioned time and time again on this forum are the Vango Nitestar and Snugpak.
I've just bought some from Winfields - they've got a range of different ones, but I was really impressed with the delivery just 26 hours after I ordered it. (Got the parcel dispatched email from DHL another 24 hours later )
I love my Gelert XL Pod (3 season) It's really roomy, very warm, has a nice snugly lining (unlike the 2 season one which is silky and can feel cold). I can't bear the thought of using a mummy style bag, way too claustrophobic for me!
------------- Bev :o)
2013
March - Pooley Bridge (Pod booked!)
May - Spiers House, Pickering
June - La Garangeoire, Vendee, France
Aug - The Cotswolds?
I'd say at least go 4 season. we do live in fair old Blighty where the temperature can range from -5° to +20°C within 24 hours. If you're too hot then you always have the ability to unzip a bit.
I can vouch for the Mountain Equipment Starlight 3, this has kept me warm in some very cold temperatures. If cold is your concern then I would look towards brands aimed at hikers, eg. Northface. Berghaus, Mountain Equipment etc.