Ron, it is just a number not a formula.

Spiky, the spreadsheet does originate from an earlier version (in fact, I
think it was originally a lotus one!) so I guess that is the answer. Not
really a problem in as much as it is just one extra item to remember to
change but would have been nice to have automated it. Maybe I need to spend a
month or so totally rewriting the whole thing.

Thanks to you all for comments