Hi Chris,
My chips never hibernated over the last few years, and I use woodsavings and hay in the nest boxes, if you feel in when they first get up the boxes always feel real warm, Only this year I think may have been a problem, but we've never had it that cold before. Hay is warmer and softer then straw, so maybe you could change to hay.
Popcorn lived at the pet shop for the last year, and he has settled in outside, and seams happy enough, sometimes we can worry to much! no matter how much you put in they always find room for them selves! and when there in for the night they push the hay towards the hole to seal it.
If your still worried you could always bring them in for a while longer, But the English weather is never predictable! it snowed last easter (I was camping, very cold!!)
Sarah