So it turns out that they're friends of my wife - who said they could park outside our house. When they arrived there was someone else in the space, but was looking to leave, so they drove onto our drive, waited until the car had left, and then moved to park outside our house. :) All sorted...
I have the registration. It was 2 women. The car is a Hyundai ix35. They both got out of the car, and then 4 minutes later one got back in and drove off. Really very strange. No idea why. The is one of the trespassers. :) I have no intention of doing anything about it, but just...