I use a diesel Clio to tow my boat. The hull weight is stated on the website as 167kg, and I reckon that with trailer and luggage and everything it is probably up aroung 300 or 350kg…so very well within the legal limit. Certainly the Clio performs as a marvellous little tractor and it all feels very effortless. I also know that the UKWA cruising secretary used to use a Renault 4 to tow his Wayfarer….850cc of raw power – possibly a bit sluggish. My guess is that your fiesta is a lot more powerful and has much better brakes that the Mk 1 Cortina automatic that Frank Dye used to take his boat and all his equipment to Scotland, but I guess also that road conditions have changed a bit so perhaps the comparison isn’t valid!