OK - since I was really puzzled about this, I wrote an email to
www.heartratemonitors.co.uk. Here is their answer:
Thanks for the mail and question about the chest belt. It is worn BELOW the bust. Normally tucked in between the bottom of your sports bra and your bare skin. Basically it should be across the sternum. You should wear it reasonably tight, but not restrictive. We do not have any complaints of them slipping down.
So tucked into a sportsbra sounds like a good solution.