what do you include in the address's in a formal.. eg, postcode, date?
Dop you mean the address on the envelope, of the first page of the letter?
The address >> Recipient's name, first line of address, second line of address, name of city, post code. one lone of space, then date.
The first page of a letter: If not on company letterhead, then your address in the top right corner.
The recipient and his/her address two lines down and on the left. First line of letter ( greeting) one or two lines below address, and on left.
First paragraph of letter: one line below greeting, and on left.
See here: Formal Letter Writing - Articles - UsingEnglish.com