THe problem from a MOD CS perspective is that staff who wish to develop their careers and thus get to the top, are actively encouraged to move posts and TLBs at least once every 2 years, so its hard to get retention when people know that their career depends on moving regularly.
As for the pay perspective for PMs - we pay a starting salary of 35K PA to a C1 CS - and this is a grade that many non faststream CS aspire to retire to! There are plenty of secretaries in the city that get paid (or at least were paid) more than this! While there are some fringe benefits to being a CS, these are rapidly diminishing - so you have to accept that once people are in need of earning a decent wage, they will move out to the private sector where they can easily double their salary. We need to look seriously at retention incentives for CS to ensure that they stay in post, and earn enough to provide the corporate memory to support posted forces personnel. Otherwise we risk reinventing the wheel once every 2 years!