Hi and Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is
Welcome to Benefits and Work Yes, you can send these letters if you think they are helpful. Don't forget to send copies and keep the originals yourself!
You might want to bookmark/favourite this on your web browser so that you can find it easily in future. This will allow you to return with further questions/comments without having to start a new topic each time. It helps us enormously when everything relating to the same claim is kept in one place - and I hope you will find it useful too