I think the 2.2 diesel has an extra solenoid valve near to the vacuum resevoir which is actually the main solenoid which controls the vacuum from the resevoir to varous other solenoids.

I've had 2 of these now which fail and cause problems with surging and often no boost.

Also on one of the cars the solenoid was very hot to touch and when in this state i checked with a power probe to see if it operated. It didnt' pass any vacuum.

Around £40 i think.