Happens on the first restart of the day only. Doesn't matter how long the car has been warmed up, the gradient of the road etc.. no commonality to it happening.

Edit: June 2019.

My local garage told me i had an ABS Control unit failure and it might cost thousands to fix, however they suggested I go see an auto spark... so i did.

Turns out that the car needed a new battery.

The car runs at 12v, but when the stop-start kicks in the voltage the battery produces drops, when the voltage gets below 9v and the foot is lifted off the pedal to engage the starter the available voltage triggers an ECU alarm and the engine struggles to start, but succeeds.

After the engine is running again the voltage is back to 12v and the problem goes away.

I had noticed that short duration stop/starts didnt cause this, probably because the voltage hadn't gone low enough. But the engine wouldnt start just as fast as when the car was new. After a period of time, maybe 30s it would happen

So they replaced the battery at 10% of the cost of the ABS Control unit makes me happy. Still not a cheap job, its a big engine to turn over and needs a big battery. Also the battery is under the left seat so the seat needs to be partially removed to give access.