This is something you could look at the easy fixes yourself, and get an expert involved if they do not fix the problem.
The sequence of leak checking is as follows (easy to hard)
Does the engine have a lift pump, if so take a look at the lift pump diaphragm as these do fail quite frequently
Take off the rocker covers, and check the tightness of the the injector cap nuts
Take off the injector clamp bridges, and check the rubber bung in the injector cap nuts is not leaking (they tend to break up over time)
Take a look at the leak off pipe and fittings, as they will now be accessible.
Remove the injection pump inspection door, and check tightness of banjo bolts on fuel inlet to each fuel pump
Check tightness of fuel pump bleed screws
Check tightness of delivery valve holders on each fuel pump
Replace Dowty washers on injector pump banjo bolts
If the leak still persists after all of the above, then your into fuel pump strip down territory (not difficult if you know what your doing).