I spoke to my HV again who has now decided that I am overfeeding him. She reckons that according to the guidelines (which say they should only have so many mls per kilo) that Noah should only be having 4 1/2 oz bottles, every 4 hrs! I thought to just feed him that would be an absolute impossibility, but we gave it a try for a few days - just to be sure.
Noah screamed pretty much non stop for 2 days, and at one point he got so upset that he started choking.
I took him to my GP, who said the HVs advice was outdated and that I should be demand feeding. He suggested putting the amount of formula UP to 7oz, and just feeding him as and when he wants it. He also said that if I wanted to try weaning at 4 months he would be supportive, so long as it was gradual and I made sure to give Noah a full bottle before any solids.
At the end of the day I guess it just comes down to what I think is best, rather than what HV or anyone else thinks. So long as i'm not endangering his health, it's up to me how I choose to feed him etc.
So thank you very much for all of your advice/opinions. I think we will try to begin weaning when Noah is 4 months old.