Autotrail fan leak 2008

[77racing]

I have an Autotrail Scout SE 2008, I have been blocking up the usual vents and holes to get rid of the cold drafts and have now come across a fairly bad leak through the centre roof forward extractor fan. If I draw the blind under the unit the drafts come out round the sides of the unit, I,m thinking it is leaking from obviously the cowl on top of the roof but can,t see what to seal up. Is there a seal missing that it closes onto ? Anyone had this problem and got an answer. My girlfriend says the draft feels like the fan is on low when we are driving along