Thanks for your suggestions. The monitor I have tells me how much electric is being used at any given time, and I've tried switching the fridge and freezer off completely this morning and it seems to make very little difference to the usage - so they don't seem to be the culprits.

There is an immersion back up for the boiler, but it was definitely off - we've made that mistake before!

I initially bought the monitor because I was convinced our new meter was faulty as our monthly payments went up from £40 to £160 in the space of about a year, but worryingly the monitor agrees with the meter - I just can't figure out what is using so much power.