I know the hv mentioned something about this, but that was months ago (and my head is like a leaky sieve). It was something to do with if you are still bfeeding at 6 months, your body/milk store of something begins to run out, and at that age your baby isn't getting enough nutrients from food to last them.
I'm (reasonably) sure the hv said it was iron, so went to my local chemist to see what they had available. There were multivitamins suitable for this age group, but nothing with iron in it. Did I just remember wrong? I asked the pharmacist, but she wasn't sure.
For what it's worth, I'm still taking pregnacare, as it's suitable for bf, and I'm vegetarian (or, more to the point, often too tired to cook properly), so my level of nutrients should be okay. The tablets have a source of iron in that, as well a what is in my diet as well.