New World, the company that manufactures the Sterling cooktops (called "hobs" in their native UK) are still in business. They don't sell spares directly for this cooktop on their website (though they do for more recent models), but you might have luck by contacting their spares department, info on which you can find here: http://www.gdhaonline.co.uk/NewWorld/Home.aspx
(I have one of these hobs, too. It's amazing how it has held up. I think mine is from 1984!)
Renee
April 2010