Finally tracked down the starting problem to a faulty/blown condenser. With the carbs ruled out, this was my last option.
Changing the condenser would be a whole lot easier if the dizzy hadn't been put near the bottom of the engine block (considering it's the one bit of the engine which will get owner-serviced). The tricky bit is having to make sure the small earth strap (which you can see in the photo running below the condenser) sits in the same screw retainer which holds the condenser, I also had to remove the condenser-coil spade connector as the one provided wouldn't fit through the whole in the side of the dizzy.
With the new part in place the MGB roared back to life - at last!