The Child Maintenance Service (CMS) is responsible for setting the overall amount of child maintenance to be paid.
The Ministry of Defence has a Memorandum of Understanding (MOU) in place with the CMS to facilitate the direct deduction of child maintenance payments from the pay of non-resident parents who are Service personnel. Under the MOU, the Department calculates a 'Minimum Drawing Rate', which takes into account gross basic pay, specialist pay, some allowances as well as gross statutory deductions such as National Insurance and PAYE.
Child maintenance deductions are always made. The only exception is when the Service person is on military operations and their Commanding Officer may advise that engagement with the CMS be delayed until their return to the UK when they are in a position to consider properly any papers from the CMS and respond accordingly, at which point deductions would commence.
I refer the hon. Member to the answer I gave her on 11 June 2019 to Question 261264 which detailed the administrative reasons for the rejection of deduction of earnings requests.