ET,
Jackonicko can probably point to some open-access sources, but in essence the 232 buy is to sustain a fleet of 7 front line squadrons with 16 aircraft each; an OCU and OEU, trials aircraft and the like (so what - about 135 airframes in use at any one time) and to provide the necessary airframes to take account of attrition and fatigue so as to ensure that the fleet can stay in service for a quarter of a century or more (more likely 35 -40 years given current OSDs for RAF aircraft...).
And having signed the contract on that basis, a contract written more tightly than the tightness metaphor of your choice, the MoD hasn't yet worked a way out of it without incurring huge penalty costs...
At least that's my understanding of the general position.