Pay Problem
NGB,
If you are having no joy with the JPAC having raised an ISupport then try the SPVA - I'll PM you a good contact who should be able to help.
At the same time I recommend you submit a Service Complaint through your Commanding Officer. You will need to clearly state your complaint and the redress you seek - not difficult in your circumstances. Do not be fobbed off by the JPAC or SPVA and stand by your guns.
Additionally write to the JPAC and SPVA instructing them that you do not authorise the deduction at source from your pay for the overpayment that was their mistake and that it is now a matter of a legal despute.
Regards
Bigtop