I was worried I had a leaking pump and did a crazy amount of research on this - ended up just being the spill lines.
If it's just the head seal at the rear as mentioned above, it can be replaced without removing the pump.
Essentially, all but one bolt is removed - this remaining bolt is backed out just enough to accommodate access to the seal. If the pump is pulled away too far you're in a tricky situation.
Old seal is removed and a new seal is replaced - there are "dieselgeek" kids with O-Rings made to accommodate stretching over the entire pump assembly without compromising form or loosening up.
Rebuild and Replace is big money unless you can somehow find a used unit - even then, no guarantee it wont be problematic. It seems any pump manufactured before the introduction of the ultra-low sulfur diesel is susceptible to seal decay.
Thankfully the massive VW TDI cult has covered this extensively.
My best advice would be to really take the time to look critically at where the pump is leaking and assess options. Tracking leaks sucks. I know.
I have a link to the viton O-Ring kit and a detailed instructional on this procedure - unsure if I can post so sent via PM
