Am confused about whether I will need one - walking from Astorga to S de C from the beginning of July - some posts say it may be cold in the higher bits - will I need a sleeping bag, or just a sheet sleeping bag?
Nights can be vey long if you are kept awake by being cold. I would suggest at least a lightweight sleeping bag. I have a Snugpak which weighs about 900gr which I will be taking for july/aug this year.
A sleeping bag, by all means. Nights do get cooler, they're comfortable to sleep on + they may protect you better from poss tiny little animals which may abound on mattresses (actually, I've never seen them, thanks to my sleeping bag). Ah, yes: try to get the smallest, lightest, one, it should fit on the palm of your hand. Buen Camino 8) XM