1) Are you absolutely SURE you have located the source of the leak? You
said you saw and felt GRP fibers, but you didn't say you saw or felt
diesel. Is it possible a fitting somewhere leaks?

2) If the layout has imperfections, the leak could travel several (or
more) inches inside the laminate before appearing. Fixing one spot on
the exterior would thus be problematic, especially if the diesel is
attacking the epoxy.

How big is the tank? How old? How is it built: Cored (eg. plywood)?
Access ports? Baffels? How is it installed/supported? Was any protective
coating applied inside the tank? Do you only have the one fuel tank, or
do you have a second separate tank that will serve if the repair gets
too interesting to complete quickly?

Given the access you have to the tank, can you do the GRP repair well
enough for this demanding application? If you're not sure, is it worth
the gamble or would it make more sense to open up the access some more,
even it it makes it harder to button it all up again? Also, given the
access (or lack), how much of the surface of the tank can you see, and
is it likely there are other leaks?

Rufus