Neff C67M70N0GB
The microwave has now gone through 3 magnetrons. They work fine for a short period and then the time taken to cook food gradually increases to an unacceptable level. The magnetrons show no signs of distress but when replaced the new one is fine and the cooker works as expected. I have ensured that there is adequate ventilation as the the engineer who changed the first one claimed that was the reason.
The most recent one has lasted about 3 months and is only used for a few minutes each day.