Many thanks to all of you!

I found that abcd's formula worked best (although I had to adjust it a
little to suit the UK version of Excel, and make the rounding a bit
smaller.

Thank you for your interesting point about the average between 90 &
270, Henry. The average can be either 360 or 180 - depending on the
direction of shift (east - west or west - east). Also, the problem of
the wind blowing from all four quadrants in one period doesn't happen
very often here in the England so I hope this won't arise!

Once again, many thanks to you all - I now have one happy weatherman!

Phil