Hi Gav, guess you still have the cover off , if so you are better to remove the cover plate that is fitted inside which is secured by a number of small self tap screws to remove these just place phillips head screw driver in slots & lightly tap head of screw driver to loosen screws. I found a lot of gunk inside this area. I believe the product you use to re-seal the plate is called "Davcon?".
Have fun .
Regards: Alan.