in the National Trust shop I decided to buy them. In the flaps on top of the instep and under the soles are small bags of wheat and lavender.
Their official name is 'Feet Warmers', but I prefer 'Hot Sox' or maybe 'Shed Slippers'...or maybe 'Those-Useless-Things-That-Don't-Work' - I shall see.
After a minute in the microwave with a cup of water the little bags are ready to be inserted into the slipper. Result: warm (and perfumed) feet. Or so I hope. I doubt that I'll be able to walk around in them very much, but since I shall be at my desk this will not matter. As soon as I have road tested them I shall report back.