Access to this drain is probably the easiest so I would suggest this is the one to do first so to know what the thing feels and looks like.
Simply remove the inner panel from the rear light cluster and fold gently back to expose the area. No need to remove the panel entirely.
Remove the drain pipe, check the drain is not blocked with a flexible rod, add the adapter and refit the pipe with the adapter into the drain.
Important to follow instructions that come with the kit …… see https://mgzt.info/2021/03/09/mg-zt-rover-75-sunroof-drain-repair-kit
More posts as a tackle each of these areas.