I know some hearty folk, Whose party jokes pretending to hunt with the Quorn

I was watching Flanders & Swann's 'Ill Wind', it's hilarious btw, but there's a line I don't get it :

I know some hearty folk, Whose party joke's (joke is) pretending to hunt with the
Quorn.

what does it mean? is "jokes" the verb here? and Quorn is just a kind of vegetable food? I don't understand the meaning.

Fyi the whole lyrics is:

I once had a whim and I had to obey it, To buy a French horn in a
second-hand shop. I polished it up and I started to play it, In spite
of the neighbours who begged me to stop.

To sound my horn, I had to develop my embouchure. I found my horn, Was
a bit of a devil to play. So artfully wound, To give you a sound, A
beautiful sound, So rich and round.

Oh the hours I had to spend, Before I mastered it in the end.

But that was yesterday. And just today, I looked in the usual place.
There was the case, But the horn itself was missing!

Oh where can it have gone? Haven't you, hasn't anyone seen my horn? Oh
where can it have gone? What a blow, now I know, I'm unable to play my
Allegro.

Who swiped that horn? I bet you a quid somebody did. Knowing I found a
concerto, And wanted to play it, Afraid of my talent at playing the
horn. For early today to my utter dismay, It had vanished away like
the dew in the morn.

I've lost that horn! I know I was using it yesterday. I've lost that
horn, lost that horn, Found that horn gorn.

There's not much hope of getting it back, Though I'd willingly pay a
reward.

I know some hearty folk, Whose party jokes pretending to hunt with the Quorn. Gone away, gone away. Was it one of them took it away?
Will you kindly return that horn? Where is the devil who pinched my
horn? I shall tell the police!

I want that French horn back.

I miss its music more and more and more. Without that horn I'm feeling
sad and so forelorn.

I found a concerto and wanted to play it, Displaying my talent at
playing the horn. But early today to my utter dismay, It had totally
vanished away. I'd practised the horn and I wanted to play it, But
somebody took it away! I practised the horn and was longing to play
it, But somebody took it away!

My neighbour's asleep in his bed, I'll soon make him wish he were
dead, I'll take up the tuba instead – WAA WAA !