In my own personal opinion Victor take it to someone who knows what the are doing. Automatic Gearbox oil changes are not all straight forward in that of emptying the old and refilling with new. I know with my old SL500 there was a procedure that was to be followed.
You will find a list of recommended garages on the forum rather than taking it to a main dealer cannot comment on Wrexham I live at the better end of Wales down South Dai Own.
Welcome to The Jaguar Owners Club and enjoy the experience.