Thanks ladies. Very interesting as always!
I am going to stick to just breast feeding for the foreseeable future. As I said, I'm really enjoying it and Sam is thriving (in fact, thriving a little too much - gone from 50th to 75th to 91st centile

). On average he wakes up twice in the night and even when it's more he's easy to settle and I have plenty of energy in the day so I'm desperate for him to sleep through.
My reasons for starting formula would be selfish - not what's best for Sam at the moment.
Thanks again for your opinions xxx