We switched to formula after 3 weeks and put LO on cow and gate, mainly because it was the cheapest!
LO was on it 3 weeks and within a few days a rash appeared which over the weeks spread bit by bit. Health Visitor said it was the heat (LO was a late June baby), I wasn't convinced.
Took her to the doctors, they said it was her milk.
Switched her to SMA and she has been fine, no rash at all.
SMA made our son very constipated, we had him on Farleys (don't do it now, became Heinz I think) which was fab and very cheap. The SMA hasn't constipated Ella at all, she goes twice a day and is on a mixture of Hungry baby SMA at bedtime and the Gold during the day.
It is trial and error!