Go along tomorrow and don't say a word about the letter

you are not lying if you don't mention it and unless they ask you directly why should you mention it? And if you do mention it that could confuse the whole situation - say nothing and blind them with your personality so they "
want you" at all costs
Just go along and ask for the chap who phoned you, you never know what may have happened is someone has turned down the job or they have more vacancies than they realised? or someone has left?
They would not phone if they did not want you? Go get 'em girl.