Hello there Lara,
In my opinion you should definitely go for the purchase of notional service and start ASAP. Cost depends on the number of years you need to purchase and your age when starting to pay. For example, if you wait till you're 40 it will cost more. Yes, it will cost more than the usual 5% superannuation but beats anything you can buy via AVCs. Reason is public service pensions are index linked unlike AVCs. Also the extra years will boost your tax free lump sum when you retire.
Furthermore, the contributions are tax deductible - 42% if you're in that bracket. You inform revenue you are buying notional service and you get extra credits on the tax cert. Sounds like you need to buy six years to bring you to full pension - half salary at retirement.
Incidentally you should check out if the years you worked in the private sector are reckonable for pension. If you paid superannuation it's quite possible they are. From personal experience, I wasn't exactly encouraged to buy notional service when I enquired which could be due to the fact that the employer also has to contribute!
Sherib