I'm creeping towards a possible explanation for this issue.
The garage say the tyres are stepped; they implied it was across the tread due to camber on the rear axle which made little sense to me (and their own tread depth measurements indicate at most a 1mm variance across the tread) but I think they actually mean along the tread (otherwise known as heel and toe wear or saw-tooth).
I was perplexed as to why this had happened this time I rotated the rear tyres rear to front and not the last time I did it; obviously different tyres are a factor but apparently a causative factor of this type of wear is steady state motorway driving, something I did less of with the last set of tyres and almost exclusively with the newer set of tyres.
I am led to believe that the heel and toe wear or saw-tooth will not correct itself now that the tyres are on the front; however as the tyres are non directional one solution that should correct it is to swap the front tyres from side to side, reversing their direction of rotation. This will reverse the previous wear pattern and (theoretically) gradually diminish the rumble.