I think there’s a bit of confusion sneaking in here. Your Calais doesn’t have a timing belt, its engine is fitted with a timing chain instead. However, if it’s a V6 engine, then there’s every chance the timing chain does need to be replaced.

This seems counter to conventional wisdom where a timing chain should last the entire life of the car. But in the case of the particular V6 we’re talking about, the chain was definitely a weak point and lots of Calais and Commodores have needed replacement chains. That was particularly true if the engine received less routine maintenance than the handbook stipulated.

The first sign can be a rattle when the engine is started; sometimes you’ll get a check-engine light on the dashboard because the chain has stretched sufficiently to allow the camshafts and the crankshaft to become out of phase with each other. Either way, it’s new-chain time and you can expect to pay something like $2000 to have the job done, possibly more.

Most workshops will be able to carry out this work and, when it comes to finding a local garage, word of mouth from friends and family is usually a good way to proceed.

Sounds like a no-brainer, stick with the mechanic you know and trust. If he's as good as you believe he will be able to service your car without a problem. If a problem arises that is beyond him I'm sure he would know to whom he could refer for a solution.

Your complaint is a common one with people who have bought the factory options, but my investigations suggest you can’t update the maps without buying the factory disc. You’re right, it isn’t good enough, and anyone thinking of buying the factory GPS should consider this cost before they do.