Anaprop, if your sole income is your Service pension the maths is simple.
Annual salary minus personal allowance times .2 will give the tax you should be paying in the year. Or the same times .8 plus your personal allowance will give your net annual salary.
Why are you paying more? Look at your notice of coding from HMRC and the coding used for your pension, are they the same? Is the coding to the pension people for the full £10,500?
Maybe you underpaid tax last year and the extra £50 is the claw back. This might have occurred because you retired last year having been paying tax at 40% using all your allowance but given the same allowance a second time against your pension.
Just get both sets of paperwork out and reconcile last year's tax and this years notices.