You can tow a maximum of 750 Kg unbraked trailer but you cannot exceed half the kerb weight of the towing vehicle.
Is it not 85% of the towing vehicle? That's the figure all the caravaning magazines and articles quote.
I wouldn't pull one knowingly of course, but I did look into the pulling capacity of my 159 before I bought my PF Jones towbar and leccy kit, to pull my Rocket, on its double-axle car transporter. It's 1,500Kg (car is a lardy 1,700)