Thanks for all the help and advice both when I've asked questions and when I've just been browsing for an answer to my latest problem-excellent site!
Thought I'd share this as I see the question crops up now and again. Saint Gobain do a strap on boss for cast iron stacks so you can connect a 50mm waste(or fit adaptors to take smaller sizes i.e. from a macerator), it's called a GT133, Travis Perkins stock 'em so they are readily available. Saves breaking ya back taking down the stack and in my case, saved me having to repair a conservatory's box-gutter after removing the stack. Anyway, thanks again and keep up the good work.