An active member on reserve forces service leave must pay their normal contribution rate, however this must be based on their Assumed Pensionable Pay (APP). See Local Government Pension Scheme Regulations 2013 regulation 13.
During any period of reserve forces service leave any pre-existing Additional Pension Contributions (APCs) / Shared Cost Additional Pension Contributions (SCAPCs) contracts remain payable (but not via payroll). The employer sends the relevant details to the reservist to pass on to the Ministry of Defence (MoD) in order to get them to arrange the relevant deductions from MoD reservist pay and for MoD to pay these over to the Dorset County Pension Fund (DCPF).