If the job is openly competed with general public you do not have to return your SPC. Only if the post is aimed at retiree/redundee is the SPC threatened.
Interesting comment. I've perused the old MoD jobs website from time to time to see wht sort of opportunities there may be, some of the 'open competition' jobs are so specific in their requirements that I could only think of a couple of people in each case, and on one, I suspect, only one.
On the other hand if it protects the SPC...