after a lot of research and help from other Elddis owners who have had same problem I have finally found out that Elddis used a sealant called Terostat MS 939 to fix the heki roof lights in place, this does not have the flexibility needed so eventually the heki rooflight will crack, you may be lucky and it will never crack or you may be unlucky like me and it it cracks after 2 years, it can be fixed by replacing and using a flexible mastic tape , I would advise all Elddis owners to check their heki by pulling out the vents and looking up for cracks ,better to check now than leave over winter