I think you do if you leave after a few years. The MOD aren't going to contact you at the age of 60 odd to remind you that they owe you money for doing 3 or 4 years service. The amount would be fcuk all but it's still money that's yours.
I'm sure some pay wallah would know.
Alternatively, ring the JPAC on 0141 2243600 and try and get some sense out of them. Let me know if you do because I can't. Useless cnuts.