Hi
The royal mail can provide this for you or the builder. If P/C is not found, find one which is nearby but make sure you have the correct full apartment address. e.g. 112 The Meadows, 10 Lawn Road, Wherever. You will need to phone Insurers who will be able to apply the details with an override. Just tell them it is a brand new apartment and that the address/PC may not be on the Insurers system yet.
huckster