Success!
I found a MountainLife Summit 300XL which is a very roomy mummy bag. And the best thing was it was on special offer at £19.99
It's a 3 season bag but, with a fleece liner it should be suitable for 4+.
It's been hard looking for a bag I like. Most of the roomier bags are down-filled. Problem is, down makes me wheeze.
I think I shall give it a test outing at the Essex Christmas do.