Glow plug problem (keeps blowing up)

I meant have only one positive feed to the bulb from no1 plug. That way the bulb not lighting up would show up a break in the circuit somewhere.

Ok gotcha yes it would

Just because your light works at end of the glow plugs does not prove the glow plugs are actually working just that you have a supply to them.

True, but it at least would indicate if any of the connections have come loose or any of the wiring has frayed.

It took me a while to sort out (took ages for the parts to arrive post Brexit) but basically the insulation on the resistor had disintegrated which was causing the problem. I changed all the wiring on the glow plugs because some of the plastic shroud was cracked and got a new resistor and it all works perfectly (for now).

Glad you got it sorted