I'm glad your car is well supported - I was worried for a while!

>>It just doesn't explain not going into L.

It could do - I know that some MB gearboxes use the hydraulic detent to also block the lowest forward gear when travelling at speed. I don't have a source of sufficiently detailled information that would allow me to pinpoint whether this is or isn't the case for your specific gearbox.

*If* you do determine that the fault is within the gearbox, there are 2 tasks to consider;

The governor is to be found behind a circular cap at the rear driving side (LHD) of the gearbox, in the tailpiece housing. This should be removed, stripped, checked, and cleaned. I think I've seen another thread on this site that describes how to do this - sometimes there's a plastic gear in there which sometimes breaks.

The detent plunger is found in the valve block, which you can get to by removing the sump, and, when actuated, the end of the valve sticks out of the valve block and interferes with the movement of the internal parts which connect between the external linkage and the main selector valve. It should move freely - however, if it has been forced against, it's possible that the valve itself has been deformed and may no longer slide freely in its bore.