Not written by Cyril. Not sure of the true origin, but certainly Mervyn sang it - ofen. Additional verses were contrived (sometimes collectively) by Mervyn, John Dawes, regulars at the Ring O Bells and regulars at The Swan. BillE's words above are most of them (they tended to vary according to location and level of beer consumption), although the Monks of Boscastle, they don't give a (H)arse-'ole - and the Monks of St. Breock were too small and we-ock, etc. One famous addition was when the Ring O Bells got so full of folkies that the verse arose:

The Monks of St. Issey were too bloody busy
To pee through the hole in the wall
And the bogs at The Ringers was full of folk-singers
So we watered the plants in the hall
The green leaved turned yellow
The green leaves tuirned yellow
The green leaves turned yellow
And so did old Charlie Carlisle

Charlie was the landlord at the time - about 1969. There's also the wonderful tale of one of Mervyn's daughters (Remember, boy, it takes a man to make a maid!), during her first year at school, when the class were asked by the teacher "does anyone know a song" volunteered, stood up and started:-

Old Johnny Bugger he lived by himself...