energy company have credited my account

[paneer]

one of the big five, not my current energy provider (or the one before my current) credited quite a large sum of money into my account.

Do I raise it with my bank or the energy company?

That's like the Monopoly card 'bank error in your favour'. :) After I'd admired the money for a few weeks (!) I'd probably start with the bank as your relationship is with them. If you call the energy company I'm not sure how they'd track down the transaction except via the amount.

I have an offset mortgage so feeling the benefit
will contact the bank next week.

I really admire your honesty btw

at some point someone surely would have noticed so best to be honest.
it is almost a year's worth of money!

honesty should be standard. Not that there is any suggestion that the OP is doing anything other than the right thing, but to keep the money would be fraud.

I'd try the energy company, possibly by email or online chat unless you want to spend hours on the phone. Or raise it as a complaint which means they get back to you much quicker.

Would it be fraud? There's no malicious intent on the part of the recipient. No wrong information has been given, it's clearly just an error. Some people don't check their bank accounts from one month to the next and wouldn't necessarily notice a big extra payment either. I'd have thought the onus is on the payee (or the bank) to ask for the money back, not for the OP to make big efforts to return it.

I would probably let energy company and bank know by email and then leave it, make use of their interest, definitely don't spend any, but I wouldn't put myself out, its their mistake and you've told them.

The way I feel about energy companies and their hideous profits right now, I'd be inclined to do nothing.

I know it's wrong and it doesn't tally with how I feel generally about being honest in business and otherwise. I guess I'd mull it over for a while. Or move it out of my account. Someone may well notice soon.

Many will credit your bank account with any money you have over paid during the course of the bill period so you may actually be due the money. Worth phoning them to check.