Thanks for responding, Nick. The formula started returning a N/A entry. But
now I have solved the problem, which resulted from a copying error: Excel was
attempting to evaluate a call using RANK with reference to an array that did
not include the cell itself. This is turn resulted from an error when a whole
line of data and formulae was copied at once by Excel, down to the line
below; so there is a mystery as to why the line should suddenly start to copy
erroneously . . . But never mind: the original problem is fixed!
Martin