Hi Siena's Mummy,
I don't really know about the eating less - I think if she's eating a nutrious, well-balanced diet without too much sugar, I don't think you should worry. Our DD is also fairly obsessed with eating when other people (especially DH and I) are eating - she will prick her ears up at the sound of the fridge opening. So whenever we snack, we give her something little, like a few grapes in a bowl, or some raisins.
I think at 17 months old, your LO is probably being quite active, so will need quite a few calories. You could always think about increasing her carbs, to make her fuller for longer?? (I might be wrong, someone correct me if I am).
But if your health visitor is happy with her weight, I wouldn't worry too much (you worrying may lead to her thinking food is something to worry about, IYKWIM). Does that make any sense?!