You also have to be careful because VOSA take the MTM (Maximum Train Mass). This is taken as the all up weight of the tow vehicle and the maximum plated weight of the trailer. It's not the physical weight of the trailer or what it's carrying. The usual work round is to have the trailer plated to the maximum your train weight can be, even if it is built to carry more. Beyond these limits you need to have a tacho fitted. A mate of mine got done for not having a tacho in his bosses Landrover when he was driving with a trailer. The trailer was barely loaded but the MTM was over the tacho threshold.