I'd recommend getting a smart meter, so you can see what's using all the energy (although I agree with everyone else that it's likely to be the electric radiators). The energy supplier will fit one free of charge (I'd also recommend switching to Octopus). You can turn everything off and check the meter's reading zero, then turn things on one at a time. Worth checking she's not overpaying, but I know from my own Mum how important it is to be warm when you're old, so as long as your Mum can afford it, and you've checked everything is as insulated and energy-saving as possible, I let her be!