what does this slang mean?

heard it twice on old tv shows now.

first on porridge, fletch says it.

then on auf wiedersehen, pet somebody else says it. can't remember who. maybe oz.

anyhoo, i cant tell exactly what the last word is but it sounds like clarks or clarts (???) or summat. if things go wrong "we're going right down the clarks" summat like that.
it seems to mean the same as going down the sh1tter.

googled it but no luck, can anybody clear this up?