The RNLI quote 436kg for the D Class. I assume thats with a 50HP on and full kit and fuel. Not sure what the 40 weighs but <100kg so even on worst case of the 436 being bare boat, and 100kg on the back you are at 536kg.
Twin axle trailer will not be light though.
UKtow shows the 1.25 fiesta (no 1.2 listed) as max unbraked 454kg so that will be an issue if its unbraked. But not sure I've ever seen double axle unbraked. 1000kg braked, although thats heavier than the car so don't be expecting to stop on any steep hills! I'd imagine is fine with your combo as 85% Kerb is 820 and I expect this trailer isn't huge so perhaps 3-400kg?
You'll not want to be going far with it, and you may be better with anything movable moved to the car..?
Don't expect it to be rushing up hills etc and go easy on those downhills.
https://www.gov.uk/towing-with-car gives you all you need to know on the licence. I assume you are a post 2013 licence. In which case its 750kg on any car or max trailer +car weight of 3500kg the Fiesta is 966kg so even at 1000kg you'll be legal.
