@Menorca Mike
Mine (on medium) takes about 100w, which is 1 kWh (24p here) over 10 hours, so would be 2.4p an hour during the day but after 11:30pm on Octopus Intelligent Go that drops to 0.7p an hour.
I put it on a timer to run for 2 hours, with the new mattress I may not need it at all. Even at 2Hrs a night a full week would only be 9.8p 🙂
My ex wife and daughter have heated throws/covers they use during the day, again only about 100w to run, last year they found them warm enough to leave the central heating off or turned right down most the day which saved a lot of money on their gas use.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!