Sleeping Bags

[Wildfisher]

My old ?Blacks Polar?  is great a 4 season bag that kept me warm in Glen Geldie one February when I needed a pneumatic drill to get the tent pegs into the frozen ground.

Fine, but an overkill for summer  camping and it has no zip which is a pain. It is also VERY heavy and bulky to carry.

Right, I need a good summer bag, lightweight  (I guess that means down) warm and compact to pack but not hugely expensive.

What should I be looking at?