Well I was hoping it wasn't a carb issue too.
First off, I swapped over HT leads 1 & 4 but still no life in pot number 4.
Then I swapped over plugs 1 & 4 but still no change, so at least I knew the plugs coils and caps were ok.
Checked that fuel was flowing from the tank, which it was, so the pump and tap were ok, so that meant it had to be a carb issue.
Pulled the carbs off and stripped them down. I did notice that the diaphragm on carb 4 was out of shape but other than that all looked fine.
Put carbs back on and it fired up on all 4, so I think the issue was with the diaphragm.
Next step is to get it MOT and then taxed.
And then, hopefully a good blast out on it!!