Further to my query yesterday about green poo I also compared my expresed milk with that of my mate's and we both reckon that the cause of the green poo is a lack of fatty hindmilk in my milk, which would explain why she went all yellow again with formula yesterday.
She's 5.5 months and takes both boobs at every feed, and always drains them both so I think this backs up the lack of hindmilk theory (it seems most babies her age only seem to take 1 boob, so her taking two suggests that she's needing that much milk to get enough calories to grow). It would also explain why her poo is green, why she poos and wees so much (after EVERY feed, almost without fail) and why she's also growing well.
So.... the thing is, how do I produce more hindilk? I already eat a varied & healthy diet (inc. porridge oats every morning).