Wow - thanks for the help with these meaty formulae! I'll try them both and
see what happens.

A further "complication" is that, although I mentioned three pairs of
columns of data (A & B, C & D and E & F), the columns are likely to extend
so I could have 50 or 100 pairs of columns! Is there any way that this idea
could be extended easily? I see that each of these solutions refers
specifically to various columns between A and F. I know that I could extend
the formulae "manually" (to include column AF, for instance) but the more
complicated the formula, the more likely I am to make a mistake and have
problems debugging it.

I guess an ideal formula would include something like <A:AF which would
refer to the whole grid of data. Logic says (to me!) that, once the
matching bit of information is found, couldn't OFFSET be used to determine
the data value one cell to the right?

Thanks again for any further input.