The usual suspect is leak off pipes or leak off seals. Have a look around the injectors, see if there any dampness. Failing that it'll probably be a pipe thats split or rubbed through somewhere. Have a good nose at all the fuel pipes and look for signs of damp or rubbing.
Thanks for this BLU. There's never been a sign of dampness around any of the injectors and I've done a basic check over of the pipes and all seems to be in order. Is it possible for a leak off seal to go without diesel leakage? Also is it possible to buy a complete set of leak off pipes / seals etc from anywhere other than Fords if they are required?
Cheers Greg