Dave, I updated the ignition system about 18 months ago: Accuspark, new coil, plugs and leads. I have checked the spark, nice and blue and juicy and it starts and restarts and idles in all conditions, hot and cold so I really don't think it's ignition. When it cuts out its a relatively slow process. I think electrical faults tend to be more sudden.
Lord13, I dont have a spare Stromberg but a replacement is my current plan B!
Mike, I'm sorry but I don't quite follow you. I supposed all BIEU are the same with a cylindrical "body" which just slides into a cylindrical hole in the piston. When it is fully in you can insert the locking grub screw which goes through the wall of the piston into the end of a slot in the side of the body, locking it in place. (a photo would have been better but I'm not in a carb stripping mood.) Mine only goes in in one position, no adjustment possible.
Regards Mike