The CS peeps who deploy seem to get rather large allowances for being there which would seem to negate the savings from civilianising the post in the first place.
Quite so. It was a practical solution in the days of a linear battlespace where contractors could be 'in the rear with the gear'. Now, in the current 360 degree threat environment, the overheads are getting expensive to keep them there. There is always the risk that they either get on the next flight home as soon as they have had enough IDF or get pulled by the company once the insurers have reached the limit of their risk appetite.
NB. Both of these scenarios have already happened in AFG and Iraq.