I’ve no personal experience, but this link may be of help: Will I have to repay the overpayment? - Turn2us
It appears to say that the overpayments are NOT recoverable if they were due to a mistake by the benefits office AND you could not reasonably have been expected to notice and tell them.
A lot hinges on the last part. Even if you didn’t realise you were being overpaid, it may be possible for them to argue that you should have done, and therefore it’s at least partly your fault for not alerting them to the error.
As it’s “only” £1100 in the course of about four years (yes, I know this is big as a lump sum), I would assume the individual overpayments must have been small - small enough not to raise any suspicion in your mind that there was any error.
It wouldn’t be something you obviously should have noticed, like knowing you’d been paid twice!
So I would contest repayment on grounds that (a) You’d always kept them fully informed, as per your responsibilities, and (b) You had no reason to suspect the payments you’d received were incorrect.
You may need specialist advice - e.g. from CAB - about exactly how to handle this, but as there seems to be neither dishonesty nor mistake at your end, or even failure to correct a mistake you should have realised, I think there’s a good chance you won’t have to pay it, or at least that they’ll accept a compromise to write off part of it.
As I say, the sticking point is whether they insist it’s something you could or should have noticed. They might say that if you knew your pension income had risen, you should have known benefits would be reduced accordingly.
But as benefit rates change most years anyway, I’m not sure the average person would know, without doing a lot of sums, whether their new payment reflected their revised income or not. Especially where the difference involved was small. It might be as little as a fiver a week, over the entire period. Could you realistically be expected to work out your payment was £5 more than you were due?