Hi Harry, As usual some answers are being given to questions you never asked and are irrelevant to your query. You are a late entrant to the Public Service and you might want to pay towards a better Public service pension. Your only (repeat) only option is to purchase years of service. This is not cheap but it is tax efficient. You have got to decide how many years you wish to buy. Calculate how many years actual service you will have up to your retirement age and then decide on how much time you wish to buy. You can purchase the time in a lump sum or through your salary starting whenever you wish, but the faster you pay off what you owe the less interest you will pay.
Is this a good investment? If you are going to survive into your eighties/nineties it is a good investment. However, if for some reason you think you will die within two or three years of your retirement it is a bad investment. Nobody can tell the future. But, if you are married, then on your death your wife will be entitled to half your pension entitlement. So I say, go for it and pay as much as your can afford.
Talk to your HR people about the rate of compound interest you will be paying. It might be prudent to put as much as your savings towards the purchasing of service.