Thanks for the suggestions.
On the '84 300D is this IP.
As you can see, there is a side cover, behind the fuel pump (removed to repair the oil leak).
There is a neoprene gasket to seal this cover.
Unfortunately, when this was fitted, one of the set screws must have caused the gasket to twist slightly, allowing the gasket seal to be tenuous at best. The neoprene has moved from the part which provided the tenuous seal, resulting in the leak. This allowed one drop every 5 - 10 seconds while the engine was running.
I have used silicone gasket maker to seal the side cover and for once SWMBO's car is not marking all of the spots on her journeys.
It was getting so bad, that the EPA were following the trail of oil spots down the highway
It wouldn't have been long and the car would've joined the dots for them