From experience (oh it is late ;-) )
STH,
You can only claim for the overnight subsistence OS ( newspaper/laundry/3 min phone call) and breakfast when you are away over night. the Daily subsistence DS covers you for lunch and dinner, there is no individual rate for lunch and dinner. As long as you are on the ground for at least 5 hours (better than the old system), you can claim DS. I think that you may also be able to claim IE (incidental expenses, £5 per day in UK, ££10 per day o/seas) as long as it is not sport etc (but worth checking).
Personally I have just been overseas on det for two weeks, and trying to complete the paperwork is a nightmare; it encourages fraud, because as numerous people have already stated, any item under £5 does not need a receipt; a few Big Mac's will do it.
If the MOD want to save money, just reduce the money that we are entitled to (when overseas as the money that we get in UK is barely adequate), and allow us to spend it as we want to.