After a short break on a long run, our Zafira's cruise control sometimes won't engage until 10 to 20 minutes after setting off again.

A Vauxhall dealer just gave the car its first annual service and said when another customer reported the same problem they replaced every possible part, with no result, and their own service manager's car has the same fault. But as Vauxhall deny there's any problem with their cruise control there's nothing the dealer can do.

Since it won't show up on diagnostics in the garage I can't prove the car is faulty, and am entirely happy with it otherwise, though I really need the cruise control because of a dodgy right knee. Any suggestions for a fix?