To start with I'd put in extra bedding, then when it gets to hit freezing or below, I'd add a snugglesafe.
PS - if you are going to buy a snugglesafe, buy it now before they sell out/put up prices, especially if they predict a cold winter. Things like that will sell more when it's cold and it wouldn't surprise me if they put the prices up a bit (can't blame them really although it's not fair to the consumer). Plus you'll have it in your cupboard ready if we get a sudden cold snap.
Compare prices online - I got mine for £12 each (plus postage) but I had seen them going for £20.