Any question answered...

A quick check on moneysupermarket.com puts an upper average tarrif in central London at £0.15p/kwh which would mean that your 365 units will have cost you £51.10. Based on current usage that would put you at £1000pa for electricity.

If you aren't using electricity for heating or water heating then this is pretty high. It could be a faulty meter but the this is a very rare event.

What sort of heating/water heating do you have going on?

Gas heating both replaced from key meters on the 28th
But no central heating in the loft conversion so an oil rad is on most nights for a couple of hours and I she's been using a fan heater during the day "to take the chill off"

I'm scared to go out and check the gas on.