BXProject

I suppose the spring could have failed thus allowing the pin a cetrtain amount of freedom to rattle

The old Moscobich used to have 2 springs that used to fail regularly. One in the carb and one in a similar configuration to yours only it was the fuel pump.
A permanent cure was to replace them with the springs from an old style telephone rest.

Oh FFS! I planned to remove the vac pump last night to see whether the clatter stopped. Started it to make sure the noise still occurred (so I could just whip the pump off and see if it stopped) and the sodding thing didn't rattle once! Wasted an hour's worth of diesel getting everything up to temperature and nothing.

Oh well, at least I charged the batteries I suppose...

I have a rather expensive hearing aid I don't use......should you be interested

Better late than never - a final (hopefully!) update. The noise was coming from the vacuum pump, which I've now changed and no more tap tap tappity tapping along the road for me. The pin which drives it was fine, so it must have been something in the pump which was worn. I'll try and open it up to see whether it's obvious.