I just wanted to back up some of what JFZ90 was saying. I've been working for a major defence contractor for a number of years now and have been involved in several projects. While I've never touched Nimrod I know a number of the people who are on the project and the principles are the same on any aircraft.
With ANY aircraft engineering project you always have the same cycle: you design something, the customer looks at it and likes it so you build it. Then the customer sees what you've built and decides he wants to change it. So you have to re-design it or source a stock of a new part that will do waht the customer wants, then amend the drawings to record the change in design, fit the new parts, test them to make sure it works and it's not a hazard or a safety risk, organising manufacture of sufficient spares for the life of the item you've just changed and finally updating the maintenance manuals. Changing just one small part of any aircraft is expensive, purely because of the amount of time and paperwork involved.
The longer a project takes, the more chance the customer has to change his mind or decide he wants something else added. What started off as a simple "Lets update the ASW platform" has snowballed and the customer wants to get more than just something to find submarines. the last time I looked there was no major threat to shipping in the mountains of Afghanistan, so why are they flying there?
Ogre