Hi Hazel,
Is your new flat in a different area to your old flat. If so your cat is going to have to learn his new whereabouts when he is eventually allowed out. If you are planning to leave him out all day while you are at work, without access to the flat. I would suggest you first let him out at the weekend while you are around, and do it just before he is due to be fed. I would go with him or follow him for the first few times he's let out, and try to see if he goes in the same direction each time. It would also be advisable to have a look around the area close to the flats, to see if there are suitable places where he could find shelter, when the weather is bad. Especially, as winter will soon be here, and already snow and frost are predicted for Wednesday this week. I don't know what area you are in but the forecast mentioned is for the North West of England. Hope all goes well.