I've asked HVs, a midwife, a doctor, a dietician and various friends. I also did a lot of googling.
ds was losing weight so I needed to know whether substituting some formula for more calorific/less filling foods would do him harm. I really can't find anything much to back up this statement. In fact, the dietician said that yes, babies should have at least a pint of formula a day, but they should be eating LOADS of solids.
Does anyone have any evidence that food is "for fun" til babies are one...?