I'm out of the system for nearly two years so I am unsure of your ground. However, it is my belief that you will have to be awarded for your previous time spent in the public/civil service. I understand the basic scale point on which you left the service previously is relevant. If that is greater than the maximum point of the grade into which you are being reemployed your new scale point will be the maximum of the new grade.
The likely scenario is that your previous pay point was not in excess of the maximum of your new grade. Therefore, you will enter the new grade at an off-point (between the scale of your previous grade and the next point of your new grade). HR will advise on which you will come on the full scale point. This will be within a year of your restart and when you're awarded the full scale point it will be from a new increment date.
You are probably confused by the foregoing. You may find that your new employer might have a different take on mine. But, if HR is worth its salt there should be no problem in advising you of what wages you are entitled. In the HSE I have seen "fast-track" processes where you would be placed onto the point of scale you were on and your increment date would coincide with your start date.
When I worked in the Civil/Public service there was a booklet available to HR staff indicating re-employment pay conditions and promotion pay processes (both relative to your case). I used to have this booklet but alas in a clear-out of unwanted paperwork I think it was consigned to the recycling bin.