Note the 550kg weight only applies to the licence needed, and not to the taxation or MOT class. The new trike is a class 4 (car) MOT, but few class 4 MOT stations will have the gear needed, so the new owners are best advised to seek a MOT station that is class 3 (light three wheelers) and class 4.
The plot thickens! We are lucky to have a 3-wheeler owner to explain the intricacies to us Martyn!
At least we'll have 3-years to get up to speed on the MOT requirements.