How to keep them toasty?

[MlleMischief]

Hello,
So tomorrow is meant to get a bit chilly here in the UK (I'm in greater London, so we aren't talking stupidly chilly, but it's going to be zero degrees celcius outside.)

Where I live is having heating problems so I'm a bit reluctant to put the heating on whilst I'm out (I'm working a long shift tomorrow and I don't want the boiler exploding/leaking/both)

How can you keep a hammy warm with no central heating or portable heating system? His current precautions include sheets wrapped around the base of the cage, two blankets over his cage(with a quarter uncovered for a bit of air) , carefresh flooring and plenty of bedding, plus an enclosed wood cabin if he wants to use it.

Is this likely to be enough (am I panicking needlessly) or is there more I can sort? As always, a nervous hammy mum appreciates all feedback!

Thanks
Xx