Hi Bryan, 

The quick reply is appreciated.  Your reply didn;'t fix it directly, but it has allowed me to see what the issue is - the report I was trying to schedule is in a "Group" and it looks as though for grouped reports only the group as a whole can be scheduled rather than the individual reports.  We had used groups to better organise custom reports, didn't realise that they limit the ability to schedule individual reports within the group. 

I can understand the logic of wanting to schedule a group of reports, but I can't understand the logic of not allowing reports within a group to be individually scheduled!

Many thanks for the assistance!