Series 3 Vacuum unit falling out

Hey clever landy folk, having an issue with my S3 109 2.6 6cyl. driving along merrily, lost power, engine slowed then conked out, coasted over to the verge and jumped out to look under the bonnet. I've recently changed the plugs (yes, all 6 of them...) so didn't suspect them. under the bonnet, I found the vacuum unit had basically fallen out of the distributor cap. I popped the dizzy off, pushed the vacuum unit back in and hooked the coil spring back over the spigot on the base plate. started up and away it went no probs, drove on another 4 odd miles, same thing again... back under the bonnet, same again with the vacuum unit lying on top of the block/exhaust.
So, first question, whats the point of the vacuum unit, and second, any pointers on what might be going wrong to cause it to want to leave its home inside the distributor cap? third, any pointers on how to keep the bugger in there where it belongs? I've tried the manual adjust (its the sort with the external adjuster) fully in and fully out, but same thing happens.
cheers
pete

advances the ignition , is base plate partially seized or tight