Dung or with custard

It occurs to me that the schoolboy joke on which that Haiku was based may well be unknown on your fair shores. It is told thus:

There was once a man with a wheelbarrow full of manure, heading towards his vegetable garden where the contents of the barrow were intended for the enrichment of the soil. His road took him past an institution for those with special educational needs. (in the days that the story was first related, this was known as "the loony bin! )
An inmate of said institution, leaning over the wall, asked the man, "What's that for?"
"I am a-going," said the man, "to put this barrow-load on my rhubarb."
"Cor!!" said the inmate, "It's you lot out there who're mad - we have custard on ours in here!"