Thank you for the replies.
Physically fitting the drive was quite straightforward but once in place, the BIOS didn't recognise it and, so, I couldn't FDISK it!!
Originally thought that the drive might be faulty - bought as new item on E-Bay. Luckily, I have another laptop and tried it in that - no problems .... FDISK'ed, formatted and installed OS. Not at all sure what to make of the Dell failure - the BIOS didn't 'see' the CD drive either. Any thoughts, please??
TIA