We are currently going through the sale and purchase process - we have been having problems with our purchase but our sale seemed to be chugging along - until today....
We have been asked whether we have ever had mice/rats and if so the location and full details of the problem.
We have, over the years, had the odd mouse problem. We usually get a pest controller out, they put stuff down, come back a couple of weeks later, replace the stuff, come back again and say problem gone. We leave the poison out under the kickboards and did not hide this from the surveyor.
The problem is that I hear them under the floorboards or I find droppings behind the kickboards in the kitchen. I have only ever seen one mouse (and one dead one from years ago when the pest controller used those sticky boards 
). I haven't had food eaten in cupboards or any sign in the living areas. It isn't a constant issue, it usually happens in winter.
The first time it happened we did a lot of mouse proofing - over the air vents, blocked holes etc but our house is like a Swiss cheese and so we cannot prevent them.
This has probably happened 3 or 4 times in 10 years. We are in a row of terrace houses and so we have found the problem to be worse as one of our neighbours does not also deal with the issue - (a newer person, most of us have been here 10yrs +) does not seem to deal with as promptly or efficiently as everyone else - I would say we had mice once in 5 years and then not again until the new people moved in 3yrs ago and since then I think it has been every year. We also live near to a pub and a curry house. There is a large park a few doors away.
I am not going to lie, I will tell our buyers, I don't think I will mention the neighbours though as I can't prove they have them or whether they do or don't do anything to prevent them (I have never really spoken to them about it, I just let all the neighbours know if we find droppings or whatever so they can also take action and they do the same for us- except the ones I suspect just ignore it).
Would this be a deal breaker for you? They have their searches and mortgage offer back, if anything it has been our purchase slowing things down and then today, this question out of the blue.
Panicking slightly that bloody mice could lose our sale.