We've always been on estimates and they tend to estimate about £50 lower than our usage so I can't complain
Nope, as I always compare my meter reading to their estimate one. Can never lose
This makes no sense!
It will always come back to the ultimate 'real world' meter reading no matter what. Estimates are just that - a pure guestimate of your usage based on previous actual readings. The company will want money for all the electricity you've used.
How long have you been saving these £50 chunks? If it's been a few quarters then you might owe them a couple of hundred pounds.
gt
Until you move and they send you a bill for £4300