Sorry Bubbleymummy, I know that growing up milk and follow on milk are different, but others on this thread don't seem to.
I made a mistake coming on this thread, because ds is under investigation for developmental delays, which is why he has a problem with eating, and why it may be that his diet will have to be supplemented in future. I have 2 other dcs and remember very well the ups and downs of weaning (dd1 weaned on purees, very fussy, now enjoys pretty much everything, dd2 blw, used to eat everything, now will only eat about 3 meals - neither has follow on milk, in fact they both like 'vanilla milk' which I make with a drop of vanilla extract and honey!)
As I said earlier, I give him follow on milk on the advice of a dietician, not because I am worried that he isn't eating enough. She has looked at what he eats, and assessed that he does need extra iron in his diet (beyond the iron provided in first milk).
I suppose it just seems very easy to look at someone buying some follow on/growing up milk in the supermarket, and think, 'why does anyone buy that stuff?' they must have been 'conned by advertising', but they may have very good reasons, that you are unaware of.