David, many thanks! That was the answer to my prayers. I'd read Michael's tip page before but didn't spot the sneaky bit I wanted:

“The opposite is true for the Scale fields, in that you can use other measurement system values if you want to scale to a specific increment, but have the results display in percent of the original. For example, you can replace 100% with 9p and InDesign will figure out what percentage was required to make it that width/height.”

Sorry – I meant to say – can anyone help?

Sorted – thanks.  It was indeed the fact that some of the ads I'd created when I first started using ID were done without Styles.  I have a House Style but some older ads had slipped through the net, causing my current problem.

Thanks for the responses.  I'll work through them during this week and let you know how I got on.