James, same as everything in the carping world, it comes down to your budget!
IMO, the best bag bar none is the Nash Wideboy Forstbite bag, will keep you warm in the coldest of fishing conditions, but at (i think) £180, its quite expensive, a mid range one to look at is the Aqua Duvet set, you get a big ol bag and a fleece cover for around £100..
If your budget is a bit tighter then the Big Snooze is a decent bag but the zips have a tendency to give way easily, £30 on a fleece cover will improve any bag immensely though fella. So maybe have a look at getting one of them to go over your fox bag.??
Ian