The Ford Zetec engine has inset plugs. Often, water will get in and fill up the hollow where the plugs sit. There are so many common faults on cars these days and often dealers are unable to repair. Jaguar have a few issues on the 2 litre diesel with poor running and their own diagnostic equipment just gives up.
VW dealer gives up even quicker and sends the owners to my boys garage. he tells them to p off and go back to VW experts. Sometimes he might spend a few minutes and fix it.
It is strange that owners go to a main dealer who is just basically putting in oil and filters and ask them to fix a fault. Ofteen, they have no idea how to fix them.
Nissan is a prime example when they were unable to diagnose a duff battery on my car. After 5 visits, they asked me to call the AA who fixed it in 30 minutes.