The bit at the bottom behind rear wheels

[liffy99]

Always an area that gets plastered with crud. What to people use to shift it?
I've tried normal washing, insect remover, engine cleaner . . . Not much impact yet.
Time to get rear mudflats methinks . . .

[Batfinnk]

Is it on the arch liner or the rear bumper?

[liffy99]

It's on the very bottom ( horizontal almost) part of the rear wing. Between wheel arch and rear bumper.

[Batfinnk]

Not quite sure exactly where you mean. Any chance of a picture?