Skipness One Echo, your post seems to suggest that the current MR2 is a heavily modified Comet-4 airframe...... All MR2's currently in service were built new from modified Comet jigs, none are original Comet fuselages. There were 2 test and development Nimrods back in the early days that were indeed converted Comets, but they have long, long since gone. One of them didn't even have the bomb-bay pannier added.
As for new-build MR4's.... I was told a story that when the Nimrod 2000 project (as it was originally known - shows just how much it's really slipped, eh?) was in the early stages and a number was finally decided upon for aircraft, that had the MOD taken another 3 aircraft they could have had brand new build airframes at the same unit cost per 'frame! Now, I don't remember the numbers but I think it was 21 ordered but 24 was the magic number for BAe to make new ones. That would have potentially opened the doors for export orders........
The current fuselages entering the MR4 conversion programme are undergoing extensive NDT testing within all the fuselage skin joints and so far all is as expected. Not perfect, but ok, was the quote I was given by one of the NDT inspectors.