A If your year head has a job description which sets out what he is expected to do, his notion of working to contract must include the duties which have been set down. Even if no job description does exist, it might be that one could be reasonably inferred from the long-standing customs and practice established in the school.
However, the limitation of 1,265 hours is not absolute. The School Teachers' Pay and Conditions of Service document allows for duties which may have to be completed outside that time-frame. This stipulation is normally taken as a reference to lesson preparation and marking,but it could also refer to those duties taken on as part of a promoted post, such as year head.
Therefore, if your year head is failing to carry out specified duties, he can be instructed to do them and, if he refuses, disciplinary action could be taken against him.
His defence can only be that the direction he has been given is unreasonable, either because it is outside his contractual obligations, or because he believes it is excessive.
It would be for the governors to determine the outcome of this case, whether that is through disciplinary or grievance procedures.