Have a look here chockhead (may only work in Internet-Explorer), use the links to the right and click the Pension Calculator.
Go through the questions, at the end on the right there are some explanations.
From the bit at the end of the Pension Calculator:
If you leave service before the Immediate Pension Point (IPP) with at least two yearsâ€™ service you will receive a preserved pension and preserved pension lump sum at age 60 for all service before 6 Apr 06 and age 65 for all service from that date. These will be uplifted by Retail Prices Index (RPI) from the date of leaving the Armed Forces. You will need to claim this pension.