Thanks cadpe
I have discovered it's something to do with "make layer plottable or non-plottable". In the drawings I am working with, which were done by an outside cad bureau,they have come back with the veiwport non-plottable. If I turn the veiwport on and make it plottable and then and turn it back off and freeze it, then save, stay with me on this one, sometimes it will plot b&w, but not always which baffles me.
The monochrome.ctb appears to have no effect on colour plotting or b&w.
I assume there is a proper sequence to follow (turning things on/off and saving), but I'm damned if I can sort it.
Any input would be very gratefully received.