Many thanks for all the replies. Lots of great advice, much appreciated.
Looking at the diagram posted by the Prof, my standard mirrors on my car achieve the blue area of vision, so I am not sure that I would gain a lot from towing mirrors, maybe see the wheels for curb avoidance, but I can see the curb and know where the wheels are.
Since initially thinking about this, I have been paying attention to caravans being towed. My conclusion is that at least 95% of caravans are significantly wider than the vehicles towing them.
Because my caravan is much narrower than average and my car is wide, in fact wide enough for the mirrors to extend 10cm wider than the caravan, I will ponder the subject some more, but my current thinking is, that the laws are contradictory and I have decent vision down the sides of the caravan, so don't need more mirrors. I may try them some day and see if the additional vision is more helpful than the hindrance of the extra width
Basically, I believe that I meet the practical requirement or the law and if challenged on the measurements, I will listen and respond as I judge appropriate.