Thanks for that - I'll store it for future reference.
Turns out there was some corruption in the Workbook! I restarted my PC and then double clicked on the Workbook to open it and Microsoft Excel gave an unhelpful error message about an out of range address segment then the PC hung completely! I got a colleage to try it and his PC did the same (but he hadn't saved all his work!!).
Anyway I deleted the file and created a new version and it all works now.