SG
"I don't think [Ilka Moor] would stand up to the variation factor.."

Well, wiki has this: 'Some singers add the responses "without thy trousers on" after the fourth line of each verse, and "where the ducks play football" after the seventh. Other variations include "where the nuns play rugby", "where the sheep fly backwards", "where the ducks fly backwards", "where the ducks wear trousers", "an' they've all got spots", and "where they've all got clogs on".

I've also heard: "...and they've all got tits on", "...where the ducks play rugby", and "...where Sir Geoff plays cricket". Sufficient variation there, surely?