Has anyone heard of an insurance policy you can take out when buying a property, which reimburses your costs if your purchase falls through?
I'm sure they exist but I don't know what the policy would be called or where to find out about them.
My main concern when buying is that the vendor may change their mind and pull out of the sale, and if this happened I want to be able to recoup the money we would have spent on doing a survey and searches etc.

Check with an insurance broker. I would imagine the premiums would be quite expensive though as the chances of a sale falling through and you claiming on the policy would be fairly high.

I believe that you can get this cover (and let's face it, you can insure just about everything in this life and beyond). Your solicitor would be the one to ask, not least because the brokers marketing this kind of insurance would be targeting solicitors relentlessly. It is niche so conveyancing solicitors would be the ones with all the gen on this.