Baby pigeon fell out of nest

I am in Farnborough Hampshire and we found a baby pigeon that had fallen out of the nest. This was on Tuesday, it looked less than a week old but fairly strong.  We put it back and the mum came back and sat with it and you could see her feeding it for the last few days.  It’s been 8 hours now and she’s Not come back.  It’s been so hot the last few days I am not sure if it’s survived.