Thanks for a neat bit of lateral thinking, Bob.

Almost there but I still have a problem - it won't update when I change font
colour! F9 doesn't update it either. However changing the 'value if false'
in the formula does force an update.

I've juggled security settings, digitally signed the module (forged
signature) and set Enable Macros on opening without effect.

Wondering if it has anything to do with putting the VBA code in a module of
a password-protected worksheet? Is there somewhere else I should have put
it?

I probably should have told you earlier I am using Excel 2002.

Alan