I've been looking for an oil leak. I had thought it might be the rocker cover gasket, but I've wiped everything clean and can't see any more coming out. Could it be the fuel pressure regulator? Well, it's a bit wet with juice but I've dried off the joints and can't see any more coming out. I wonder whether it just got a bit messed up when it was in the garage recently and they had the fuel pipes off and were testing fuel pressure, had the rocker cover off to get at the injectors, and so on. Maybe it's just residual mess, and when the engine gets hot a bit more drips off. I shall keep an eye on it and see what happens.