Scuttled,
There are people on here who know far better than I do, and I'm pottering around at home so don't have the DIN to hand. But, IIRC, it is based on both how much time you have done and how much you have left.
Time served in years x redundancy rate (based on how long you have left) = your redundancy payment.
If you have passed an immediate pension point, you get the redundancy payment on top of the pension and gratuity. If you have yet to hit your IP point but have done enough time you get a resettlement grant on top of the redundancy and I think a preserved pension which becomes payable at 60-ish. Outside of that, I don't think you qualify for anything other than the redundancy.
So there is probably a point at which it becomes too expensive to sack you because of the up front costs they will need to pay out. Without doing the maths, I have heard that if you have 4-5 yrs to run i.e have a reasonable amount of time under your belt and a couple of tours to go, it becomes quite costly for them to get rid of you as that redundancy rate mentioned about is quite high.