It's all in here
The Armed Forces (Redundancy, Resettlement and Gratuity Earnings Schemes) Order 2010
From what I can see for those with more than 3 years to serve and after an immediate pension point it is as follows:
AFPS75: tax free 6 months pay plus tax free gratuity and then normal pension.
AFPS05: tax free 9 months pay plus tax free gratuity and then normal pension.
Watch out though, all changes to 3 months pay on 1 Apr 2013!
LJ