Learn English – Where did “You know what thought did!” come from

Matt's idea that the phrase might have a different meaning in each saying seems probable to me.

I've been slightly more successful searching for see a pudding crawl on its own (without the laughing part). And it is a lot more frequent with creep than crawl.

There's a peak in the early 19th century for see a pudding creep because it was used in an essay by Jonathan Swift :

it would vex a dog to see a pudding creep

where it could be understood as "see a pudding go to waste".

The oldest quote I have found is in a 1617 nonsense anonymous verse :

I grant that Rainbowes being lull'd asleep,
Snort like a woodknife in a Lady's eyes;
Which makes her grieve to see a pudding creep,
For Creeping puddings only please the wise.

Here again it's the idea of waste. So how did it become associated with laughter? (A hint of sadism maybe?)

The phrase seems to be British (rather than US), here's an excerpt to what seems to me to be a pastiche from the TV series Star Treck:

"You would laugh to see a pudding crawl..." Spock read aloud, an eyebrow almost rocketing off his face. "...a fascinating image."

Kirk gave him a weird look. "I never thought I'd say this, but that's one of the only phrases I haven't heard Bones say."

"That is hardly surprising, Jim, as it originates in Britain."

It can found in a cockney dictionary and a 2005 excerpt from a web blog:

But then, us Londoners, as my dad used to say, would laugh to see a pudding crawl.

The phrase is Irish in origin but now very rarely used in Ireland (except as a sterotypical "Irishism"). It simply means "the best of the morning to you" - perhaps from the idea of unhomogenised milk, where the cream rises to the top. An appropriate response might be a simple "thank you" although the traditional response would be "And the rest of the day to yourself."

Terrible attempts at Irish accents, dancing a jig and leprechaun costumes are entirely optional while saying this.