That's what I thought AoS. Got to do a return beczuse of the changes to child benefit. Actually there is a section which allows for expenses of this type but I'll try for the DS answer from the pay office before submitting
Army allowances were always considered as taxed at source in my day and had no place on a tax return. That was the deal arranged between HMRC and the MOD to avoid the ballsache of paying more Govt dosh out and then taxing it.