As expected, filter was blocked up with crap, but nothing metallic, oil was filthy, sump plug was "welded" tight - in other words what has been said about VW style gearbox servicing. Because of the foregoing, they ran a sacrificial load of oil through to flush as much muck out as possible. Once finished, the box performed to spec & didn't throw up any nasty messages. Phew!

And on the positive side, they were carrying VW engine oil so they did a service as well.