I think the main thing to remember with the MRA4 is that the wings are completely new - and thus so are the integral fuel tanks. So in that respect, the same problems should not occur. Assuming new sealing techniques have been developed since the original Nimrod design of course.
Fuel pipe leaks are a different matter, but again they should be of a completely new design, so I don't think it's fair to compare the two aircraft in regard of fuel leaks.