I now buy M&S long sleeved nighties and put them over Mothercare PJs (on offer at the moment). I was wondering the same and didn't want to go back to vests with poppers underneath at night to keep her tummy warm. What about this?
Get 2-packs of soft jersey leggings from mothercare, sainsbury's etc (2 for £5 or similar) and 2-packs of basic nighties. Can't see why it has to cost much at all. Look through mothercare/sainsbury's bargain rails too - they always have copious ones, not just at sale time.