Testing a solenoid valve. It failed. Shouldn't be letting water through when there's no power applied. Hence the leak in the Italian coffee machine, that it was removed from. It's specialised to the extent that only the original valve manufacturer's ones will fit. They're also Italian. Managed to track down a couple of UK sources. The first only dealt in large quantities, and wouldn't help us. Were rather snooty about telling me that. The second could get one from Italy for about £33, which is fairly reasonable. The post and packing was a fair bit... and then the chap contacted us again, with an apology, as it was doubled to £22 for Highland, with their courier. And all before VAT, of course.
After that, I dismantled the valve to see if I could fix it. Alas, it still leaked. Did note from reading the coffee machine's instructions, that it should have a filtered water supply (it didn't), otherwise warranty claims would be not be accepted. And right enough, the problem did seem to have been caused by the thing slightly furrying up inside.
Information passed on. Others will decide what to do.