If you accept substantive promotion there is a 2 year return of service. Have a look in the AP it's all there; it implies that a PVR within 2 years would only be accepted in exceptional circumstances and you will be asked to sign a letter acknowledging this fact before you accept substantive promotion. The alternative is accept acting rank only, turn down substantive and then do 2.5 years to get the pension.
You never know, hang around for a few more years and they might just make fg pay pensionable or b....