Wombat - Are there weep holes in your pump? If it has holes and leaks from there, it is the seals. Pinpoint the leak, clean off the pump, run the engine, and watch for the leak. If it leaks from the gasket between the block and pump, no big deal. The rear seal could also be blown, but its harder to tell because of the location close to the block. If you replace the rear gasket and it turns out to be an inner seal, it will find a way to leak again. If the leak comes from the front, where it connects to the impeller housing, it is the pump seals. Engine oil coming all the way from the block out the front, you either need to rebuild or install a new one. I don't trust rebuilds. I installed a new one and rebuilt the old one for a spare. If you take the pump off, check the shaft for tightness in the bearings.
So, first locate the leak, if its from the rear mount replace the gasket first. If it is from the front, you have a blown seal and there is oil where it should not be, so rebuild or replace. For a rebuild, hire a pro, peace of mind.
Also, check this site for info:
Torresen Marine's Marine Diesel Direct :: Index The Sailboat Specialists