Having had a good poke around, I discover that about 50cm back from the numberplate, dead centre on the underside, a rectangular black nylon block sits in a recess moulded into the very extensive shielding.
Now without removing the shields cannot see what it attached to but I think it must be bolted to the underside of the crossmember which runs under the front of the engine. The 320cdi engine takes up a lot of room but you can see the ends of said crossmember on either side and it seems in the right place.
If I wanted to put a front central jacking point on a car which is otherwise pretty much plastic clad on the whole underside that is where I would put it.
Can anybody confirm this or tell me it is something like a cover for the sump drain plug?