It's come up before on here. You should be aware that there is a difference between the MOD/RAF definition of duty travel and the HMRC definition.
Even if you get an answer on here I would seek guidance from the tax office (not SHQ, because, through no fault of their own, they are not the best authority)
Should be able to clarify this via a tax office call centre (or maybe these days doing an online search?)
I claimed £600 back in 96, and 3 years later it was challenged. After a very aggressive investigation, and a mountain of paperwork, I got a stroppy letter saying I had claimed falsely and I had to immediately repay.........£10.