CJ,
I had a similar experience on return from the TELIC sandpit in 2007. I was over paid an Op Allowance by #7630, when I enquired, I was told that it would be taken back from my pay at 4 days pay per month! Great, a 14 month interest free loan. Wrong! The next pay slip arrived minus half my pay. The Minimum Drawing Rate is 50% and that's not 50% of what's left once Tax, NI, Accomm, RAFBF and Lottery are taken into account. Oh no, it's 50% of what's left after Tax and NI. Therefore, if you do pay all the other stuff, you'll end up with less than half pay. The argument being that you've already got the money in your bank account to cover the overpayment. Unless of course you've already gone and bought your shiny new KIA Ce'ed!
TBP