It's a funny rule and we looked in to it a few months back and the law is not too clear, to the point where most coppers are not completely sure either. In the end I spoke to a Motorway/pursuit copper friend who went away and clarified it.
There are 2 rules that apply. The theoretical gross train weight, e.g. max plated weight of trailer and car combined must be less that whatever you are limited to on your licence, depending on when you passed or did a trailer test and actual weight of the trailer and load must not exceed the max towing capacity of the vehicle. E.g. the polo can pull the trailer if it is
a) less than the cars max limit and
b) trailer and car are plated less than total gross train weight on the drivers licence.