Sir Topham you are right ..... and wrong! DA @ £21 is to cover lunch and/or dinner and/or breakfast if not covered by NS. If you are out for 5 hours and choose to have a slap up two course bar meal and non-alcoholic drink costing £25 for your lunch you are quite within your rights to claim £21. Any blunty that suggests otherwise should be put back in his box firmly.
You are wrong to suggest that one could claim 25 x McDs @ 99p because that would not reasonable as a lunch or dinner. Those who seek to circumvent the regs with too many sub £5, no receipt, claims will have the no receipt clause revoked and will have to produce everyone whatever the amount.