Tony, trust me I've cracked sleeping bags, I've tried all sorts of combinations and never been comfy until I found the perfect combination.
You buy two, one very cheap and one expensive. The cheap one I have is a double bag from go outdoors and it was a tenner; the expensive one is a vango viper 1000 down bag.
The skinny twats amongst us can get away with just the viper and less packing space but lets face it, like me, you're a big chap, in fact you're bigger than me and I can't fit into a normal sleeping bag.
I use the viper unzipped as a quilt inside the double bag on a double airbed, it's the best combination I've found yet, it's bliss! For added comfort take proper full size pillows. I can get all that in one roll bag and if you get the bigger roll bags you can get your tent in too, what's not to like?
If it's hot you can kick off the down bag and use the cheap one only, if it's freezing you can zip the down bag up for extra warm and still be inside the double too, although you sacrifice some comfort but you're not cold. I have not yet needed to zip it up though.
