Selling a property with historical rat problem

We are wanting to sell a cheap mid terrace house in the North-East of England. About three years ago I became aware of the nasty issue of rats in the cavity wall.
Fortunately the entry point was identified as broken drain in the neighbouring property and this was fixed by relining the drain and since then there does not appear to be any more rats.
Now we want to sell and since 2013 I know that the seller is obliged to reveal to a buyer anything which is not immediately apparent with the property which may influence a potential buyers decision to buy.
-like rats, of course.

So what I am trying to gauge, and would be grateful for any ideas about, is if I tell a prospective buyer about the historical rat problem what effect is it likely to have on demand and price.
Any ideas about  if i am obligated legally to tell given that we have not been bothered by rats for three years now since the drains were repaired and so there should be no practical implications for any buyer.

we have not been bothered by rats for three years now since the drains were repaired and so there should be no practical implications for any buyer.

You've answered the question yourself!

It's just the same as:

"I had a leaking roof due to several roof tiles that had cracked. I replaced all the defective ones with new. The roof no longer leaks"

You surely wouldn't be questioning whether you need tell the buyer about this.