GBZ : I think there would have been some changes with the MR4 but I would not be able to tell you what. Some of the other guys on this web site might be able to answer your question. I left regular service in 75, worked on the Mk3 for two point five years and then the VR back on the MR2 from 78 until 94.
ShotOne:
How long is a piece of string. Most of them at Kinloss in the mid 90's were dripping fuel everywhere and I heard quite a few were limited to 55,000 lb. One or two had been lost , there was usually between four to six in Flight sheds at BAe Woodford being converted and chopped about and three were at 51 sqn. I would hazard a guess in answer to your question - at its peak the nimrod fleet could probably field twenty five ish aeroplanes capable of flying with 84,000lb and generating a 9 hr sortie and spread out between Kinloss, StMawgan, Wyton, And Luqa u(the latter until 1977). Maybe Wyton is not part of this question.