All contractors in the Construction Industry Scheme (CIS) must send a form CIS300 to the Tax Office by 19^th of each month, to report payments made to subcontractors. If you haven’t paid subcontractors in a particular month you must still submit a form CIS300 showing nil payments, or ring the CIS helpline on 0845 366 7899. If you fail to submit the CIS300 on time you will get fined £100 for each and every month (or part month) the form is late. We can help you appeal against these fines if you had a reasonable excuse for being late.

 Guidance for contractors on monthly return penalties

http://www.hmrc.gov.uk/cis/returns/late-return-pen.htm