Hi Smuj, and others.
We were told on the OCU (originally) that the bank limitations were derived due to the movement of fuel outboard during (uncoordinated) turns. This 'slug' of fuel (depending on the severity of the yaw).
I'm trying to find the info, but I read of an incident where this 'slug' of fuel caused complete failure of the outer wing box and the fuel broke into the vent box (if I remember correctly - bear in mind I've been qualified on 6 aircraft types since). I'll come back if I find it.
Indisputably the bank limit was 60˚ clean and 45˚ with flap.