Many thanks for your comprehensive reply, extopia.
"I think it certainly used to be the case that if one was non resident and had no taxable income arising in the state ..then no tax return was necessary."
That was always my understanding also, but according to what extopia has written above, even non-resident directors have to complete a form 12 if they satisfy either or all of the three conditions. Is that right, extopia? Sorry if I seem to be labouring the point, but i need to be sure about this one, it could make a big difference to the directors of the company.