Tracked the problem down to a single type of calculation that fails,
even in the debug screen.

Try entering in immediate mode:

? 123 * 456

Any easy ways to avoid runtime error 6 with sums of this type? I don't
want to start creating variables, constants etc in a section of code
that performs a long series of simple pre-set calculations that need
to be 'human readable'.

Thanks everyone!

Using Excel 2003 vba.