I think what I would do is get myself a Vango Astral 350XL (which is a big bag) and then get a liner for it. That way you have a sleeping bag you can use year round.
I also find that decent night clothes are essential and although not glamorous, fleecy pyjamas are essential in winter as sometimes one needs to get up in the night (if you get my drift) and we have a van!!!
I use a Nash Frostbite Extreme Bag. It has plenty of room (I am 6' 3"). It is very warm and can be used with a breathable waterproof outer for sleeping outside.It is about 3 times more bulky as a normal sleeping bag. It is made for Carp fishermen who can snuggle down at night waiting for their bite detectors to go off. Try Googling Nash Frostbite Extreme.
Vango Nitestar 300 XL, it is a extra large single bag at 110 cm wide all the way down and it is also a 3 season bag so suitable for those cold nights, I think you may be able to zip two of them together to make one massive King size bag.