I've seen the talk about other appliances but no one seems to have talked about cooking........is this by gas or leccy......If it's leccy then that can have a very big impact on a domestic bill.
Immersion heaters are a killer on the leccy too as are fan heaters ect.
On the 'Being in credit with your supplier' front.....I rang up my supplier to find that I am, wait for it, £555 in credit and this was after they had sent me a letter saying that I ought to up my payments slightly.......I told them they ought to..........well I think you can guess
I pay around £90 a month for leccy and gas and I have a 4 bed house with big mental PC, old and greedy tv, fish tank, 10.5kw electric shower which gets approx 1 1/2 hours use every day,washing machine and tumble drier both of which are used at least 3 times a week in the winter months 2 laptops, ps3, Wii, and other various games consoles and we are in credit with both suppliers....
If it were me I would A, get a power monitor or B, get the leccy board to check the meter is accurate and not overcharging.
One other thing to check is that the bills are not done by estimated usage.....get a current reading from the meter and call the provider and check it is up to date.......If they do an estimated reading they always 'Go Large' and if this has happened on the last few bills then it is going to have a snowball effect.
