@NoSquirrels you said that you have had no issues with insurance, but then you haven't had to ask them to pay out any insurance on your actual house? If you had, and you knew about the risk of flooding when you bought the house but hadn't told them, then they could legitimately refuse to pay out at all. If you had told them about any known risks, ie your garden in this instance, they might well have increased your premiums to a very high level.
@Welikethemoon is it possible for you to ask any home insurance providers about how much they would be likely to ask for in premiums, for a house similar in age and build to your potential one, and one that has also had the same amount of flooding as the one you are contemplating?
Also, do you know of any land engineers (sorry, I don't know the correct terminology you this, I don't think it would just come under arquitect, as I think it would have to be a more in depth - no pun intended - study) who could advise you on what, if anything, you could do to stop, or at least drastically reduce any future flooding of the property?
I think the answer to both of those questions could well show you and your 'reluctant to pull out of this sale' DP, that there is no sensible choice, you will need to forgo this house. I think that this is probably why the seller instructed her Estate Agent to tell any potential purchases up front about this problem, as she did not want this to happen at this later stage. She has probably, and presumably, already spent money on her next purchase (unless it is the sale of a deseased person's property).
Good luck OP, I hope that there is a not too expensive way around this problem, and that you do get to buy your - in every other way -very suitable house. 🤞💐