Access free from the Standards for Highways website. Also available from CIS.
The requirements and advice for the management of maintenance on the trunk road network are described in two documents, the Routine and Winter Service Code (RWSC) and the Network Management Manual (NMM).
You should read the introductory parts to each of the documents to understand their coverage. National Highways updates them regularly to reflect changes to policy and is developing methods to publicise these changes.