We use the Get Address search functionality to search for addresses, and to pre-fill the form for you. This has most, but not all addresses included. If you live in a very new build, it's less likely to be in there.
The number you use can be either the flat number, or the building number.
If it doesn't find your address, first try searching with just your postcode, and an empty number field. It will give you all the options that we have for that postcode and you might be able to find yours in the list.
If after that your address is not available, you can type it in manually in the boxes below.