750kg is the max for an unbraked trailer if the car is big enough to tow it. You are only allowed to tow 50% of the weight of the towing vehicle so the car would need to be 1500kg. The rule is "
Maximum 750kg gross trailer weight or half the towing vehicle's kerb weight - whichever is less." Pull 750kg unbraked with your Fiesta and you'll be pooping when you have to brake sharp going downhill. I wouldn't advertise that my trailer towed like a dream at 70mph either as it's a 60mph maximum and no fast lane on the motorways and the police do like to enforce it. I'm cautious with mine and have fitted a heavy breakaway cable since a few years ago when a child in a pushchair was killed and mother seriously injured by a runaway trailer not far from here.
See section 4 of your vehicle V5 Document (vehicle details), about half way down it states the towing specification of your car both braked and unbraked.
