Charlie,
I know Nick's brother had a similar prob with his Scubby - it was the oil seal on the end of the crankshaft - I think it only leaked when running.
I also had a similar issue on the old Golf . But with the water pump - the thing lost water when being driven but was OK when parked - I though it could have been the head gasket as no water was ever left on the drive. But in the end I lifted the bonnet with the engine running - when it got hot it sprayed water from the bearing in the pump, but as soon as the pump stopped with the engine it would not leak.
Rob