I did consider clamping the pipe to see if the vacuum was leaking away through the actuator or somewhere else, but couldn't get in there with Mole grips with the intake hose in place. But it does sound like it's all operating as it should even if it thinks it isn't and is bringing the MIL on. Can't do any more on it now for a couple of weeks as it will be setting off for Devon very early tomorrow morning but I'm definitely going to crack this as it's bugging me. It'll need to be in a state where the MIL is off when MoT time comes anyway. Oddly, I drove it with the MIL on and even cruising at 80 mph, it still seemed fine and with the new owner driving it at 60 mph if it wasn't for the MIL showing you wouldn't know there was anything wrong at all.