A new twist - it turns out that the dealership have messed up the fitting of the fuel pump! When under load, usually around 4,000 RPM, the car will suddenly stutter/cut out. It will also randomly ’flat spot’. Both of these are typical symptoms of fuel starvation. Apparently, when changing the pump, there is an O-ring that easily falls out and this seems likely to have happened here. When this happens the pump works but the pressure is reduced. Anyway, it is back at the dealership right now and hopefully they will be able to sort it out!