Thanks to all for your replies. I used time, pulling effort and WD-40 on the 4in tape that needed to be removed. Then trimmed all old excess dicor from around the shower sky-light, cleaned all areas with acetone (very sparingly) wiping it off quickly. Re-sealed using Dicor and as of today there have been no further leaks. Since finishing we have had three pretty fair rain showers, plus I've washed it twice and so far all is well.