We don't have pollen filters in Canadian Foci, but I got curious as to how they could be causing trouble on British models. My official Ford factory Focus manual (purchased in 2000) has a section on it.
On page 412-01-04 there are two interesting notes:
"A grill with rubber edging is inserted between the wiper motor and the filter housing. This must be cleaned of dirt every time the filter is changed."
and probably more important
"NOTE: Ensure that the air cowl grille basket for the windscreen is seated correctly. Clean the gasket if necessary".
The word "NOTE" was in caps as well as bold. In service manuals, this is a signal to the mechanic of a potential trouble spot. So my guess is that the leak is likely to occur when this is not followed.