In reply to George Fisher: I use an ex-army gortex bag (£40) to add at least a season of warmth plus cuts out any drafts. It weighs more than a liner but I've always found myself tangled up in them. It looks weird in a hut or tent but works a dream. Normally with a 3-season hollofill bag; this set-up dries clothing extra well.
I once had the water bottle leak & soak foot of sleeping bag & socks. Hitched up my feet & all was bone dry in the morning.
In more extreme conditions my Paramo is silky soft so I can climb straight in then remove & use as a pillow when it's dry & I'm overheating.
Down bags also benefit from being in the gortex despite my worries that the down wouldn't loft properly; wouldn't try the wet clothes trick though.
There's always a moment of waking & rolling over when colder air reaches parts you don't want it too. Best solution I've found is to get up, have a pee & get back into what now seems a toasty bag once more. zzzzzzz