children's book about monsters in the toilet???

[midnightexpress]

One of my friends is looking for a book from her youth (late 60's/early 70's) featuring a little boy who (she thinks) goes down a toilet and finds a land full of fairly psychadelic monsters. Does this ring any bells with anyone, and if so, what was it called?

Can't help I'm afraid but I'd steer clear of it when potty training.

Will bear that in mind if we manage to track it down.