Final episode (I hope).
My son's workmate gave him a DVD writer which he said worked, but wouldn't work in the case for some reason. We took it out of the case and it looked the right size, so we removed the old CD drive, installed the DVD writer (screw holes were in just the right place, though we had to google to find out how to take the front bezel and front of the tray off) and put it all back together. Switched on and got the old blinking question mark. Back into the Mac to check connections and noticed the slave/master pins were set differently. I told my son to move the plastic connector along to the next pins, reassembled it, switched on and it reads and writes DVDs. My son's happy as he's got an upgraded iMac, and I get loads of brownie points for thinking about the little bit of plastic!
Many, many thanks for all your contributions.