Finally tracked down the vacuum leak on the turbo in the car, the vacuum pipes were like a plate of spaghetti, I have now rerouted them all so I can more easily work on them in the future, what the hell Mercedes were thinking when they built it is anyones guess. Anyway, I replaced a couple of hoses as they were old but nothing was leaking, turns out it was the MAP sensor on the manifold, I am guessing the membrane is perforated in the sensing module as the fault seemed to be more likely to happen on wet days, I had previously thought it might be electrical but I checked, cleaned and waterproofed every possible connection. Seems it was being affected by barometric pressure. It must have been feathering the waste gate actuator as well as now it has a lot more boost and I can hear the turbo spool up sooner. Luckily I had a used MAP sensor in my spares drawer.