What got to me was that I spoke to the person who did the work and he never mentioned the fact that the horseshoe clip on the pipe had snapped - just said that the fixed pipe had shown up other boost leaks and that they recommended I get them to change all of them. I originally took the car to them for diagnostics, but their scanner couldn't talk to my car, so the work they did was work I could and should have done myself and I'd have fixed the snapped clip and not had to go through six months of trying to track down an issue, replacing parts as I went, when I should have checked the boost pipe first.
When the oil had cooled down, it was back to the normal black colour. You're probably right about its age, and given I can get the Quantum oil for less than £30 compared to the Mobil 1 I was using before that was near enough twice that, it's much less hassle to just do an annual change.
All in all though, it's nice to have my biggest
problem sorted so I can move on to more exciting stuff.