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.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.