Jacko- the Harriers have mostly done something like 3500-4000 hours in twenty years. Some have done considerably less depending on the role they have been used for. The potential life of the aircraft depends on flying hours and the type of flying they do. The vast majority of aircraft that have been converted to GR.9 have had life extension mods carried out. The idea that they all need new back ends is rumour and only that. The RAF has spare rear fuselages for a very limited number of aircraft which require the change but that is nothing related to fatigue.
So in summary the RAF Harrier fleet doesn't need £1 Billion pounds worth of mods to add another five years to it's life - it's not serious to suggest that GR.9 upgraded aircraft that leave the Jump line next year will only have four years of life left in them.