whereabout in my ba can i find my cabin filter as its getting slow to clear the windscreen recently so more than likely it has never been changed haha. If someone can let me its location and difficulty in changing id be very grateful

Down in the foot of the passanger side.Behind the glove compartment.. there is a cover with 2 screws.
Also, your F may not have a filter.. try to find the cover, if there is no cover with screws,well you have no filter..

Any other reason why it'd take so long for the windscreen to clear

Internal coolant matrix may be leaky, or you have a lot of moisture inside the car..
Try to heat it up to the max that you can with blower on max T and speed on spot.
Go inside some underground parking(mall) and open all doors when hot even the boot.
Try some cleaner for mirrors from the inside, and than one of those antifog sprays they sell on the shops..
Works every time..
The reasons are many,
-clogged drain lines from sunroof(if sunroof is presence)
-bad seal/s on door/s
-leaky coolant matrix
-bad isolation of the body pannels
And so on and so on...
Hope that helps.

Ya that's perfect thanks