Not as silly as it sounds, Laurence. You CAN get 12V automotive water pumps. It could get you back on the road!
Most seals are made like that shown below. If the seal in the water pump is the same (I can't remember if it is or not), it'll have a extension spring, circled, & wound into it's tail. The head will be slightly tapered to make this possible. It's this spring that keeps the seal firmly in place over the shaft it's sealing.Hmmm.... that's sounds like an interesting alternative to replacing the whole water pump. I'm pretty sure that the shaft is the culprit as the seal around the spring was in pretty bad shape when it was replaced & the shaft needed a good clean. What do mean by decreasing the ID? and do I refit it tighter?