Mother left baby pigeon


we’ve been enjoying watching a nesting wood pigeon look after it’s baby in a tree by the end of our garden in North London. Today the mother hasn’t been in the nest at all and with the weather being so hot we are concerned for the baby. We’re unsure how old it is. I don’t want to disturb the next unnecessarily but also don’t want to leave it to die. Any suggestions to what I should do?