After more testing with central heating completely switched off, I've found that when my blood sugar is extremely low (not eating for 18 hours gets blood sugar extremely low [intermittent fasting]), a 40W electric blanket quite doesn't produce enough heat, when the room is at 13 degrees centigrade. It'll have to be the 70W king sized version for the garden. I've turned the heating back on, testing is over for now... but I'll still use this 40W electric blanket and sleeping bag to reduce my gas bill, and to thoroughly warm my toes, because they love it.