That is wrong information.
Richy if you passed your test after 97 which 2001 is way past then it doesn't matter what you think you can tow, you can only tow up to 750kg.
If the trailer is 500kg unladen then you add 500 or 750 or 1000 litres of water you are going to be over your weight and driving on public highways, unless you fill up and empty on private property.
It has nothing to do with the difference between the unladen weight of the van and the gross weight - you can only tow up to 750kg full stop - until you do a trailer test.
Rob