Do we have to declare this as a seller?

[Horcrux]

We have a manhole in our garden it is for the communal sewer pipe for our side of the road.
Over the 5 yrs we’ve been here it has overflowed twice and we have had the water company out 3 times to pump out... which happens at another house garden further down.

Each time it has happened it hasn’t been us who has reported it.... we only realised it had overflowed when water company knocked on the door to inspect. Our neighbour next door is reporting as his overflows before ours.

It is happening because neighbour keep putting baby wipes down the toilet.

Do we need to declare this to prospective buyer?... On the one had I am pissed off that the previous seller didn’t tell us, but on the other, as a seller it would jeopardise sales!

The fact you have a public sewer in your garden will come up at some point during the paperwork process of your sale so there is no way of keeping it from your potential purchaser IMHO.
I don't know if you would need to declare that it had been unblocked 3 times - presumably, yes if you are asked.
The good news for a purchaser is that the water company is responsible for unblocking and sorting a public sewer - other folk have to sort out their own drains on their land once it's left the public bit.
We have a public sewer in our side return (terraced house).
Never put me off.
Never had any problems with it.
Will use it as a positive, if ever we come to sell 😊👍

Depends what you mean by declare, you wouldn't put it on the listing, but if you make a false statement and the buyer relies on that, the buyer could then sue you for whatever damages there were, if any.
If they specifically ask about it you could either give the facts or just say no comment/no information.
To be honest our manhole has never overflowed but it has blocked twice and I'd never even think it was a problem, let alone something to warn buyers of up front.