Took this panel off , to replace said filter !!.. When I replaced it ( fiddly job) there was a gap between the top edge of the panel & the windscreen . This will allow water to enter the footwell . Mechanic at work , says once removed , there will be a gap . Seems to be an inbuilt fault . Anybody come across this .

Yep it is a poor design!

When you buy the new filter it normally comes with a strip of double sided foam tape that is supposed to reseal this gap. i used that and some silicone sealant to be sure. if it does leak the first stop is your heater blower motor then the passenger footwell. if the air from the vents start to smell "musty" it is an indication that the filter is getting damp / wet.

H

On the mk1, the filter housing does not seal to the windscreen - it will not keep water out. The problem arises with the panel which is attached to the underside of it, it's job is to divert the water to either side of the filter. But water can get through the join in the panels.