The original parody was found on www.lyricsdownload.com, posted by someone called "TheUndertaker180". I've adapted it to fit the CI5-verse.

 

THE 12 DAYS OF A CI5 AGENT’S CHRISTMAS REFRESHER COURSE

On the first day of Christmas old Macklin gave to me, a cartridge in the left knee. (Ow!)

On the second day of Christmas old Macklin gave to me, two swollen bollocks and a cartridge in the left knee. (Ow!)

On the third day of Christmas old Macklin gave to me, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (Ow!)

On the fourth day of Christmas old Macklin gave to me, four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (Ow, fuck off, not again!)

On the fifth day of Christmas old Macklin gave to me, five broken ribs, (An’ here we bloody go...) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (Put the gun down, ow!)

On the sixth day of Christmas old Macklin gave to me, six broken toes, five shattered ribs, (I didn’t fuckin’ know he was hiding there!) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (The training’s worse’n the bleedin’ job!)

On the seventh day of Christmas old Macklin gave to me, seven wounds a-bleeding, six broken toes, five shattered ribs, (He's a right nutter he is!) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (Here it fuckin’ comes!)

On the eighth day of Christmas old Macklin gave to me, eight compound fractures, seven wounds a-bleeding, six broken toes, five shattered ribs, (‘Oi, that’s my fuckin’ gun!) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (I’m in fuckin’ agony!)

On the ninth day of Christmas old Macklin gave to me, nine solid knockouts, eight compound fractures, seven wounds a-bleeding, six broken toes, five shattered ribs, (I’m gonna get my fuckin’ grenade launcher!) four flying headbutts, three chinese burns, two swollen bollocks and a cartridge in the left knee. (Surprise, surprise!)

On the tenth day of Christmas old Macklin gave to me, ten total hammerings, nine solid knockouts, eight compound fractures, seven wounds a-bleeding, six broken toes, five shattered ribs, (Oh fuck, Towser’s coming now!) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (I’ve had a titful of this!)

On the eleventh day of Christmas old Macklin gave to me, eleven lacerations, ten total hammerings, nine solid knockouts, eight compound fractures, seven wounds a-bleeding, six broken toes, five shattered ribs, (Oh fuck, the old Cow’s got a gun, too!) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the left knee. (How many fuckin’ bullets has he got?)

On the twelfth day of Christmas old Macklin gave to me, twelve days in traction, eleven lacerations, ten total hammerings, nine solid knockouts, eight compound fractures, seven wounds a-bleeding, six broken toes, five shattered ribs, (Don’t fuckin’ stand there, ring an ambulance!) four flying head-butts, three Chinese burns, two swollen bollocks and a cartridge in the fuckin’ left knee. (Surprise, sur-fuckin’-prise, fuckin’ hurts an’ all. FUCK OFF!)

Merry Christmas, everyone!