Thank you for everyone's reply.
ReverseEMF your last comment has led me towards a resolution.
I substituted A0 with A15 and I was getting all 5's (1023). I am using an official POE based Arduinio Ethernet Shield 2, it seems I might have a loose connection. When I plug in to say A15 and use a GND based on the board itself it works as I hoped.
I will sort it out from here but want to say thanks to everyone, you have all been very helpful and generous with your time.