Whirlpool AMW848/IXL Microwave not heating/cooking well.

Hi All – I was seeking some advice if possible with regard an AMW848/IXL Whirlpool microwave.
I had been noting recently that when cooking (usually at 900w) the time taken to obtain usual results was increasing – Eg – Broccoli would normally steam in 4 mins, but now takes more like 7 or more mins. It seems the microwave has lost some of its power.
I heated a litre of water at 900w for 1 minute, and found the temperature rise was only about 3 degrees (suggesting a cooking wattage of about 200w, not 900w)

A lot of information available online relates to a microwave that has stopped heating entirely, but little info about ones which are not heating so much. Is this a symptom of something other than a failed magetron that someone here is familar with (eg a defective thermistor/temp sensor – )

There are no error codes showing at the moment (though I do recall an “F” code once pop up which went away on a power cycle – but for the life of me I cannot recall the number – anyone with a list of codes which could job my memory) –

Finally, do these microwaves have any hidden service menu I can check?

Many thanks in advance.