The quick answer
The main dealer gave me a long explanation of how the engine needed to have warmed up and the ambiant temperature had to be above a certain point before the stop start would operate. When I insisted it had never worked he suggested they could look at it under warranty but if no fault was found there would be a charge.
I shrugged and gave it no more thought. Then one day I changed just one setting and it began operating perfectly.
What was it?
I always had the air setting on demist and because of that the air conditioning was engaged by default. I simply pressed the A/C button to deactivate it and that was it, the stop start worked flawlessly.
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