Split radiator hose
A quick look in the engine bay after a drive when I spotted a bit of water on the ground revealed the source. There was water on the splash panel inside, but it wasn't the radiator. Turning the engine on to help pressurise the system showed it was coming from the radiator hose where the clip had cut through the rubber. It was an old hose and I think I'd just tightened the clip too much after refilling , plus I tend to disconnect this hose at the stat cover and pour new coolant in when refilling which probably stressed the old rubber in just the right place.
Replacing the hose did give me the chance to use my new tools - a full set of coolant pressure test caps which I'd bought to test my daily driver's cooling system. I'd made sure I bought the larger pack which included a cap which would also fit the B's expansion tank. This meant I could check the new hose and clip had sealed by pressurising the system via a pump attached to the adapted expansion tank cap from the kit.
After a slight leak was fixed by tightening the clip, I left the pressure set overnight and checked it in the morning. Pressure was holding so everything - including the new water pump gasket - was holding fine.