Technically it is English, as 'Seppo' is Cockney rhyming slang: 'Septic Tank' rhymes with 'Yank', 'Shut yer north and south' rhymes with 'shut your mouth', 'on the dog n' bone' rhymes with 'phone.' And my favourite: 'Trouble and strife' rhymes with 'wife'. Not so much insulting as playful. Some makes much less sense. 'To have a butchers' at something comes from 'Butchers hook' which rhymes with 'look'. So you see, it is actually alot of fun.
My grandmother-in-law here in Australia is a 90 year old Cockney who can rhyme up a storm. She is brilliant. My wife, who is a dinki di Aussie (as far as I know, 'dinki di' is a fair dinkum Aussie expression) can give a good run at rhyming slang, but she is left for dead by her Nan.
Cheers,
Josh