Utilities companies which are paid by fixed sum standing orders, can, if they review and find you've been paying back more than you've used, refund you. They do usually tell the customer, but it might be this, in which case it's your money.
On the other hand it's likely to be a simple mistake. If it is, and someone realises you absolutely have to refund it. However, if it's down to someone else's error, you are entitled to keep any interest. I'd bung it in a high interest savings account. If no-one's asked for it back within a year an a half (to make sure you get past the point of the year end audit) you can probably get away with keeping it. Efven then the rightful owner can ask for it back, but if you've spent it you're entitled to make reasonable payments by installment.
The reason I know all this stuff? Both of these things have happened to me. In the latter case it was enough to pay for a car!