Looked up in the spring upper mounts today and sure enough, there is a rubber pad about 0.25 inches thick stuffed up there each side.
Phoned my local MB dealer and after a lengthy search on their 'puter, they said for standard suspension cars they only come in one thickness, but the guy was unable to say what it was.
The sports suspension versions apparently have 3 or 4 different thicknesses but again the guy didn't know what these were or if they were suitable for fitting to standard suspension models.
They didn't have any in stock and take about a week to order up, costing £4.79 each + VAT. Without knowing whether the new ones would be thicker than the ones I have, I didn't bother ordering. Guess I'll have to try other suppliers...
Maybe I could get away with fitting two or three sets of 0.25inch pads to raise the car?
BTW, The front springs have a peeling red paint on - I guess this indicates they're aftermarket springs, as surely MB paint wouldn't peel would it?!