When you make a cover for your overflow, make it about a inch bigger than the overflow. That way it will shade the slots on the outside.

Marie, I had a simialr issue and used the back edge of a small paring knife to scrap it out from between the teeth. Worked well, just watch the sharp side.

Thats what i'm doing now, It's very fiddly though and the durso pipes keep getting in my way

Is the coraline growth on the inside of the overflows? If so, I agree about the covers.
Maybe something like this would help?:
http://i23.photobucket.com/albums/b3...3/IMG_0807.jpg