try st michael's church, ha9 6sl. You'll need to ring to book a space, but it's just off the north circular, close to the A40, about a 10 minute walk from the stadium and no hassle to get away from. Cost £10 for the semi. Apparently they usually open up toilets and put tea on in hall as well, but there was a function on on the semi day.
I used the official car park for the semi, but the silly cunts sent the pass via email on an editable word document rather than posting a hard copy (which they are doing again for the final), so if anyone wants a copy, PM me with your email address. All you have to do is edit the date and car registration and you will sail straight in gratis