Thanks - permissions on /dev/fd0 seem ok.
Progress of a sort. The man page for fstab-sync is utterly incomprehensible, but it refers to a HAL daemon. On the start up of Fedora it gives the option for interactive stary, where you can skip any step. By taking this route and disabling the 'load HAL' step, but leaving everything else as it is, I can now mount my floppy.
This is not a permanent solution - interactive start up is slow, and if I once avoid it I will have to re-edit the fstab file. But if I disabled the HAL daemon, would I notice it was gone? What does HAL do, anyway?