In reply to tomasis:
hmm, many choices.
I might ask friend to make a quilt using loft Apex from Climashield and fabric Argon. The price should land on £60.
PHD overbag is very tempting and I opt it as last choice if everything doesn't work.
I've been eyeing on cheap Lamina minus 15 from Mountain Hardwear at such bargain price though it is a bit heavy 2kg. I don't believe in their temp ratings, it seems overoptimistic, doesn't it?