I joined aged nearly 25, on the old scheme. After 20 years, I decided to do something about the missing years, so asked how much it would cost. The answer was "a lot". I opted to buy 2 of the 4 missing years, which then cost me something like £350 a month (a long time ago). However, it then got more complicated when I got to 55 and was offered an extension of service. I served the missing 2 years, and then more. The small print about buying back missing years clearly says that any such payment is not refundable, so I both served and paid for some of the years. However, it was still worth it.
I then compounded matters by first taking maximum life commutation when I left, then joining the RAF Reserve, first on an ADC, then as FTRS. Leaving aside the immediate hassle of my leaving process and the redress that failed to get back all the promised payment, I am still left with having to pay back the commutation, which has proved quite complicated. So the pension is very nice (and rose by 5% this year) but the process of getting it is not simple.