Electric issue?

I doubt you would get carb icing in the UK in July.

When it dies, is it very sudden or is there some momentary rough running? Sudden failure is more likely to be electrical, fuel problem might run rough then die.

Process of elimination

You gotta start somewhere, right?

I would do this, simple & cheap. Use 12v buzzer. Have it in the cab with you. Connect one end to ground; connect the other end to the coil + terminal. When the problem happens, if the buzzer stops, then you are losing 12v to the coil. If ok, next step.
One of the ballast resistors is 5 ohms; one end of it goes to the 5 pin plug on the ECU. Connect the buzzer wire to this terminal & check for loss of buzzer. Buzzer might sound different, that is ok.