Fixed sum expense allowance

My client has paid a fixed sum of £30 a night to staff when working away. Luckily they haven't had much work away in the past year, so the exposure is minimal. The company pays the B&B costs direct and the £30 is to cover meals, refreshments and out of pocket expenses.

I've tried to agree the treatment with the local tax office but they consider that £30 is too high, so I have to either justify the figure in more detail or suggest a lower one.
I know that they should only reimburse actual expenses plus £5 out of pocket expenses, but my client wants to keep his existing system as the staff prefer it.

Does anyone have any experience of how much would be a reasonable daily amount? I have a vague memory of receiving a fixed sum allowance in a previous job and I think that I read that the Revenue or some other govt dept use a similar scheme for their staff.
Any suggestions would be gratefully received!
Thanks
Susan H