Jackonico- Again the GR.9 programe has not been run to provide the RAF and Navy with an aircraft that has a shelf life of only 3 1/2 years from the last one to leave the line. Similarily the extension of life directly reflects very much based on the fatigue life and hours on the airframe. With some aircraft at 4,000 hrs and others a considerably less it doesn't take a genius to deduce that they will not all be life ex at the same time.
A sensible choice would be to reduce the number in service by say 30% and turn the high hr machines into spares/war reserve. As for replacement rear fuselages -seven from memory have been made by BAe - the number made is in direct relation to how many are actually needed otherwise it would have just been a random number plucked out of fresh air.
So again whilst your IPT contact might have been pesimistic -don't for one minute think that 1 Billion of your hard earned taxes is needed to keep her in the air past 2013.