Apologies in advice for this being an easy one, but I seem to be having a
dumb day!

I have a conditional format in cell M17 which is a 'Use a formula to
determine which cells to format' =$M17<$K17 full red

This works fine.

I am then trying to copy this condition down several hundred lines, but it
copies it exactly as =$M17<$K17 rather than changing the row number each time
it is copied.

How do I copy it so that it changes the line number every time?