For employees on busines travel ,you can pay, based on receipts (tax free) or you can pay HMRC benchmark scale rates, (tax free).
Which is dependent on the time they are working away for.
See here for more information on HMRC benchmark scale rates :
Exemption for amounts which would otherwise be deductible: payments at a benchmark rate
Or see the 490 booklet here:
Tax and National Insurance contributions for employee travel: 490
This is for the employee side only, for CIS please contact the CIS helpline.