I have not towed with a Fiesta, but having a quick browse, there appears to be two parts to the answer:
1) Your Fiesta does have a 950KG max towing weight (according to http://www.cuddles.abelgratis.net/ford.htm#fiesta305), but:
2) Unless you have a very fancy road base, it will be unbraked. If it is without any braking, it is classed as a Type 1 trailer. The rules (Law) appears to state a maximum trailer of 750kg gross trailer weight or half of the towing vehicle’s kerb weight. Focus on whichever is less. In the case of your Fiesta, this would be approx 475KG.
Hope this helps