Well the first thing you should check when having heating problems is your thermostat. They are notorious for going out in our cars...
Along with that you should look at your water pump... if you haven't replaced it go ahead and do it, but make sure your new one has metal impellars. Its a cheap buy and you'll have to do it eventually.
I don't want to make u feel dumb but have you checked your coolant levels? Hoses? Do you have any leaks???
If your mechanic was right, though, you need to take your car to a specialist shop for European cars and have them check your cylinders because we can't help you out too much online!
Hope that helped