The problem was me ...
My .fluxbox/startup file has:
I had somehow accidentally removed the: &
Therefore the start up sequence for Fluxbox was not progressessing any further than xclock. After putting the '&' back again, things are now working as expected.
My .fluxbox/init file still needs to be configured to customise the third screen but currently, as I type, all three monitors are displaying separate Fluxbox desktops, which is what I want.
I don't know if there is a: 'Weekly LQ Dufus Award' but if there is, I would like to apply and at the same time apologise for this thread.