Also if you retire early, the NSP may be actuarily reduced. In my case I am buying some notional servise WRT 60 and another batch WRT 65. If I go before 65 I will he hit twice. The acturial reduction before 65 and you wont have also made all the payments you set out to make.
Also important to note that the core system pension modellers are not correct (in my organisation anyway) if you are purchasing pension WRT 65, and you have not purchased it all yet, or indeed will get an actuarial reduction on it as well. You just need to be careful.