Having applied 'Read Only' to a spreadsheet in Explorer and attempting to open it I get a message saying that it cannot be found. Un-ticking 'Read Only' allows it to be opened normally again.
Sorry for delay in responding - I've been away. I only want to make these spreadsheets Read Only to prompt other people using them not to alter the original but to save as copies when data for specific jobs has been input. Will follow up your suggestions and let you know if I get the right answer.