Found it! Here's the full text:

'Twas Christmas day in the cookhouse, and the place was neat and tidy.
The soldiers were eating their hot-cross buns - I'm a liar, that was Good Friday!
In the oven a turkey was sizzling and to make it look posh, I suppose,
They fetched the battallon baraber, to shingle its parson's nose!

Potatoes were cooked in their jackets, and carrots in pants - how unique!
A sheep's head was cooked with the eyes in - it had to see them through the week!
At one o'clock "dinner up" sounded; the sight made an old soldier blush:
They were dishing out Guinness for nothing, and fifteen got killed in the crush!

A jazz band played in the mess-room, a fine lot of messers, it's true,
We told them to go and play Ludo, and they all answered "Fishcakes to you!"
In came the old sergeant-major, he'd walked all the way from his billet;
His chest was turned out, his toes were turned in, with his head back in case he'd spill it.

He wished all the troops "merry Christmas!", including the orderly man;
Some said, "Good old sergeant-major!", and other said "San fairy-ann".
Then up spoke one ancient warrior, his whiskers a nest for the sparrows -
The old man had first joined the army when the soldiers had used bows and arrows -

His grey eyes were flashing with anger, as he threw down his pudding and cursed:
"You dare to wish me 'merry Christmas'? You just hear my story first.
Ten years ago, as the crow flies, I came here with my darling bride,
It was springtime in the Rockies, so it must be the same outside.

We asked for some Christmas dinner; you gave us pease pudding and pork.
My poor wife went to the infirmary with a pain in her Belle of New York.
You're the man who stopped bacon from shrinking by making the cook fry in Lux,
And you wound up the cuckoo-clock backwards, so now it goes 'oo' 'fore it cucks!

So thank you and bless you and b - low you; you just take these curses from me:
May your wife give you nothing for dinner, and then warm it up for your tea;
Whatever you eat , may it always repeat, be it soup, fish, entree or horse-doovers;
May bluebottles and flies descend from the skies and use your bald head for manouevres;

May the patent expire on your evening dress shoes, may your Marcel waives all come uncurled,
May your flannel shirt shrink up the back of your neck and expose your deceit to the world!
And now that I've told you my story, I'll walk by myself to the gate.
And as for your old Christmas pudden, stick that - on the next fellow's plate!"t

It's in a book called The Book of Comic and Dramatic Monologues, compiled & edited by Michael Marshall & published by Elm Tree Books / Hamish Hamilton, London, England - ISBN 0-241-10738-5 or 0-241-10670-2.

There, Steve