The dealer has had the car since Saturday but didn't get round to looking at it until today! Thankfully, it was just the sensor that needed replacing, not the pump.
Didn't realise a faulty sensor could cause the engine to cut out.
After prancing around in buses and a hire car (new shape fiesta), it feels fantastic to get back into the GTI. You appreciate the sharpness of the controls, the secure feeling of the seat, and the speed, my God the speed!