Richard,
As pointed out by Lutz in other postings, the 7% guidelines came from the days before noseweight data was supplied by the caravan manufacturers.
Since the introduction of type approved towbars, both the tow bar and the caravan tow socket are now stamped with maximum permissible limits.
The legal requirements are that:
1. The car + caravan + you and your family + your stuff must not exceed the posted "Gross Train Weight" in your vehicle documentation pack.
2. That the caravan + your stuff not exceed the maximum permissible towing limit for your car.
3. That the noseweight not exceed the lower of the caravan or car posted maximums.
There are other legal requirements if you have only passed your driving test recently.
Organisations like the caravan club "recommend" that drivers new to towing only consider a caravan that is less than 85% of the weight of the car. This seems to be very good advice.
Robert