Hi again
After much toil and sweat I've found the answer.
In the 'OLE DB (ADO) Connection Information' screen my sql server name was in lowercase. All the other reports that work fine had the sql server name in captials. So, against all logic because my SQL install is a case insensitive installation, I browsed on to the sql server name and chose the sql server with the name in captial letters.
That has solved the problem - imagine that!
I've posted this solution as maybe it can help someone else.
Chow
Sheila