With the B having issues occasionally and having replaced a bunch of parts I wanted to ensure that at least the basics of timing and carbs were set correctly. Luckily I managed to find Alf who is a local old-school mechanic engineer running a local tuning shop and is a dab hand with carburettors.
Alf did come up with a couple of things. Firstly my coil connections were swapped. No idea when this happened, it could have been when the AA chap was switching out the terminals, or when I changed the coil at the roadside (though not being confident around wiring I tend to be super-careful), or it could have been like that since we pulled the car out of the garage years ago. It should hopefully only have manifested as a smaller spark, but if recent could have been a contributory factor.
The other thing he found was a slight leak on the front carb butterfly, likely due to the disc not seating quite right. I don't think this was the reason for any of my issues as I only rebuilt the SU recently.
He also said my timing was only out by a degree or two, which I'd say wasn't bad for a driveway job.
So with a confidence in the underlying stuff I can be sure that if the issue resurfaces it isn't in the timing or the carbs.