Slightly extended and re-arranged:

DISCWORLD NIGHTFALL
Nigel Parsons (tune: Skyfall)

This is the Broken Drum.
A place we watchmen like to come.
My brain is turning numb,
Because I'm drinking Scum – ble

This is a friendly pub.
But bring a crossbow, knife or club.
I really should have had more grub.
I'm getting drunk, and here's the rub

When it's nightfall,
I drink Scumble.
I would stand tall
If I, If I could stand at all.
At night fall.

There's a man we've never caught.
He travels at the speed of thought.
To him a closed door's just a port.
They tell me that his name is Mort

And at nightfall
He drinks scumble
Just like his master does,
But the drink doesn't puddle at his toes

Fred Colon wants a rumble
Angua wants a tumble.
Me, I shouldn't grumble.
I'm getting pissed drunk on Scumble.

'Cos at night fall,
I drink Scumble.
I would stand tall
If I, If I could stand at all.
At nightfall.

So I've been drinking Scumble
It makes me feel quite humble,
Mumble, mumble, mumble,
At nightfall Zzzzzzz. . .