If you're reasonably mechanically-inclined, there's
a whole thread on the forum at
www.vwdieselparts.comIt shows a complete tear-down of the injection pump,
with pictures,and seal replacement.
Although the Canadian Bosch people seem reluctant
to sell the gaskets kits, for these injection pumps,
you can get a complete kit (Bosch Part#DGK126)from
Bosch Diesel specialists in the U.S. Cost about $20US
I get my kits from a company in the States.
The guy I deal with is very helpful.
Your "leak" sounds like the "O"Ring between the
cast-iron head and the pump body.
Fairly common, not that difficult to fix. Just a little
fiddly to get the head back on.