406 TD Oil Leak, but where from?

My 1997 406 1.9 TD has developed a terrible thirst for oil. It's recent
history includes a refurb head after the cam-belt snapped and shattered the
cam into three pieces!

It was ok immediately after being done, but now is losing up to 5 litres
every week - almost as if something has worked loose.

The oil seems to originate from somewhere the back on the passenger side
(RHD) of the engine, near the vacuum pump. It runs diagonally down the
water pipe at the back of the engine and then drops directly onto the
propshaft, with the result that my floorpan will now never suffer from rust!

I thought the o-ring seals in the vacuum pump were to blame and replaced
them (again). Indeed, on removing the vacuum pump, it seemed that the leak
was visible on the lower, smaller outlet, but the problem remains...

Is it possible I have fitted them wrong? (I am an amateur and no-one seems
keen on teaching amateurs to suck eggs these days). Also, I couldn't find a
torque setting, so just did them 'reasonably tight', then after more oil
loss, a bit tighter - could they be too tight? Or am I barking up the wrong
tree?

hope someone can help, 'cos the car is reasonably splendid besides...
cheers
Hurricane Charlie.