4000 electric @ 50p a unit (which maybe where it's going) is £2000 a year
7200 gas @ say 10p a unit is £720 a year
So that's £2720 plus standing charges - let's say £1 a day - so £365
Roughly £3000 a year. Deduct £600 credit. You are also going to get £400 from the Government - so maybe £2000 a year will be your bill
£2000 / 12 = £167 a month
And that is why I pay monthly on receipt of bill
(But you need to know the actual unit costs)