Thanks for your suggestion.
I suspect the floppy is used in a standard way driven by the interface attached to it.
As I have never seen it running I cannot record anything on the discs.
Alarm 10 comes up most of the time "Disk Write Protected".
There is no 5 volt supply to the FDD or to any of the chips on the interface so I suspect that this is the reason it does not work.
I have been trying to find a schematic so that I can investigate properly but no luck up to now.
It could well be that the CPU board sends signals to turn the 5V rail on.
When I can have another go at it I will get my oscilloscope on various pins to see what happens.
I have tried once but it didn't tell me a lot.
Next time I will use a dual trace scope and see if that helps.