As much detail as I can find:
Added Pension
As a member of the Scheme you can pay personal contributions in order to purchase Added Pension to increase either your retirement benefits and/or dependants’ benefits. You can choose to pay either a lump sum or fixed monthly payments over a scheme year. This is a contract between you and the Scheme Administrator. You will need to take out a new contract for each scheme year that you intend to purchase Added Pension and any Added Pension contracts taken out part way through a scheme year will cease at the end of the scheme year and will be shorter than 12 months. A new contract will need to be taken out at the beginning of the next scheme year. Only one Added Pension contract can be taken out per scheme year.
The amount of Added Pension you purchase will increase annually in line with the Consumer Price Index (CPI) in each subsequent scheme year until the pension comes into payment. The amount of Added Pension a member can purchase throughout their public service career will be subject to Her Majesty’s Treasury (HMT) limits. The total amount of Added Pension that can be purchased over your public service career will be capped and from 2015 is £6500 but could be subject to change in the future.
You may cancel your Added Pension contract at any time but you will only receive a proportion of the amount of Added Pension you would have received had you paid the full contributions. If you leave the Armed Forces before you have completed 2 calendar years qualifying service, you will receive a refund of your personal contributions, less any relevant tax charges.
From FPS:
Currently, the maximum amount of extra annual pension that someone can benefit from is £6,500 per year. This sum is set by Her Majesty’s Treasury and may alter. The minimum amount you can pay towards Added Pension is £300 per year as a lump sum or £25 per month. We have not seen the factors yet but, as a rule of thumb, about £1000 paid at the age of 30 buys an extra £100 of annual pension for the member. The factors, when available, will cater for the variables such as your age when you start to pay for Added Pension, whether you pay monthly or by lump sum and whether you buy Added Pension just for yourself or yourself and your dependants.