In reply to the sheep:
Frankly it's bad news and it's only going to get worse. I've lived in my current house (OK it's an old watermill) for ten years and the natural beck from which water was channelled to power the mill is actually about 50yards away but it seems that every year it gets not only more expensive to insure but also harder to find companies that will even consider it.
As has been said, insurers use the flood risk map but certainly the map for this area has changed in the time I've lived here - and, needless to say, not for the better. Partly that can happen just from people getting more cautious but also developments upstream, over which you have no control, can affect the situation too.