OK chaps, had a think about this one now, heres what to do to confirm for any leaks. I have not worked on a T5 for ages now but do tell me if this tandem pump is on the end of the camshaft. If theres nowt there you must have an electric pump in the tank.
Anyway check the colour of your fuel oil filter, now unless you use really mucky fuel, it should not be inky black, if it is then you are leaking sump oil into your diesel. Try to take a dip of your fuel tank and see what colour it is, if its inky black then you have contaminated fuel returning back to the tank from leak-offs.
If so remove your fuel filter and pour it all into a container. Traditionally its black with muck but this time it might be black with sump oil. If thats the case, then check the seals within the tandem pump (if its camshaft driven) . The black diesel will be contaminating all your system so if you have it you will have to remove tank, drain it and clean it.
Naturally fit a new camshaft fuel pump (or fit new seals) and new fuel filter. This fuel pump normally only is used to fill the diesel gallery around the PD injectors, its set to about 15 psi but may rise to 25 psi when engine is flat out. Make sure pump-to-head gasket is sound.
Those big O rings around the PD injectors are where you get diesel into sump oil, nowhere else....these are the only 2 sites where diesel and sump oil are next to each other........unless that feckin' head gallery has a hairline crack or it porous.....!!
Again thats why I drive MB.
:*:*