I had a May baby. Her first week we had such a cold snap that meant DH headed to mothercare to buy a pram suit (happily in the sale. Two weeks later we had a heatwave and she spent most days in a short sleeve vest, or just her nappy. Sorry that's not much help is it!
I worked on the basis of a vest and a sleep suit generally. I had cardies but rarely used them.
I had an April baby. Although on the day he was born it was quite mild but we brought him home in a vest, sleep suit, Cardi and blanket PFB , the few weeks after were very dull and cold. So he often went out in all the above and an extra blanket. But then we had the warmest June for a while so he lived in nappies and vests for a few weeks . Good old British weather.
Good advice I was given-always put them in one extra layer that you would wear.