Worked a treat! However it felt safer to get the trim off without breaking anything by easing it up by the side closest to the top edge of the tailgate after having started it on its way by doing what you said, i.e. "there's a piece of trim along the top this just pulls off i find pulling it from the bottom near the window is easiest". I cleaned up the 5 rubber seals on the spoiler's mounting feet and smeared them with a little silicon before putting it all back together. Hopefully it has cured the leak.

Thanks again.