Hi haukex,

Thank you very much for the links: they are incredibly helpful, and I will be sure to follow the guidelines when I post in future. I would never want to come across as some kind of freeloader! I contribute to quite a lot of specialised forums in other subject areas, and I know from experience that it is incredibly frustrating when someone comes along and appears to be taking advantage of the knowledge of the forum members.

I will now update my original post with an edit that reflects the working code. Thanks again for your kindness and advice.

Lisa