I am on a local forum for old gits and was sent a link to get some 4.7 Tog cosy socks. They are very thick so you need a size 10 rather than a 9 to wear them in slippers.
Someone then suggested a few blobs of bathroom sealant on the soles to turn them into 'gripper' socks.... and it works! Three pairs for £10 and winter is sorted.