The N16 has quite soft suspension anyway, and with that much weight im not really suprised its sagging.
Not trying to teach you to suck eggs and all but the maximum braked towing weight of the N16 is 1200kg (not sure what a Minor weighs). And from memory the nose weight maximum is 110kg.
Maybe have a check on the nose weight to see and also have a look through the brian james spec sheets to see if the towball height on the car is in spec for the trailer. The hitch can be spaced accordingly if not.