The guy did answer!
"Greetings

Thanks for your mail.

I've always assumed "to laugh like a drain" was onomatopoeic, i.e. imitative of the sound of water gurgling down a drain. I don't know if it's British (perhaps British and Australian) but it's definitely not just Welsh. A search on Google might help to pin down where it's used, or you could visit Michael Quinion's site WorldWideWords.org and see if it's listed there.

Best of luck

Harry Campbell"
******************************************************

Found your limerick, Sweet One! I had an idea of the first line that turned out to be right.
There was a young lady named Kitchener,
Who slipped on the quayside at Itchenor.