Cooling systems are pressurised which means they can run hotter without boiling. If there is a split in the header pressure will be lost and whilst the engine will probably be no hotter it will boil and probably alert the driver to a fault especially when the level falls below the sensor.

Thanks all so far. The dealer thinks that the new reservoir might be fitted next week and I'll find out if it has fixed the issue with some driving around afterwards.

In the meantime, the fan has been working overtime these past couple of weeks, so I know it works. There's still the question about the thermostat though.

At present I can't use the car as the short drive home from the dealer drained the reservoir completely. I'll refill it for the drive back next week.

If the expansion bottle is split and leaking as I said above it will cause issues but you don't need to be a mechanic to spot one of the issues, there will be a very noticeable stain from the leaking coolant.

Is there such evidence?

The new plastic coolant expansion tank was fitted today. So far so good - on the 30 minute run back from the dealer in the slow rush hour crawl the temperature gauge stubbornly stayed in its normal position.

You can't knock the Hyundai 5 year warranty too much - the car is 4 years and 9 months old and they reckoned the faulty part was covered at no cost to me.

Result!! Glad to read your problem seems to be solved, so many post about a problem and are never seen ( heard from?) again, many happy miles of motoring to you.