But the simple question is, how do I contact the wordpress user "streathamswradio". Does wordpress provide a messenging/email system to contact other wordpress users?

Especially if that person's not even using it. If they're leaving comments pretending to be you, that is something you could take up with a lawyer. If the are, those will show on Google. If they're not, there is no problem.

In any case, staff don't provide contact information and no volunteers in the forum would either as far as I know. You could try google. But if your blog is the correct URL, I wouldn't worry about the username. It's not like somebody's going to mistake their nonexistent blog for yours. If you really want to tweak your username, you could create a new account with some variation of the name like streathamADMIN or something and make that an admin of the radio blog.

I want to do this because they are using an id that I would like to use.

I use "streatham216" wordpress user id to run a radio station blog streathamswradio.wordpress.com But someone else has registered/created user id "streathamswradio" - I want to use that so that so that the blog name "streathamswradio" is the same as the user id.

I don't want to use the option invite as administrator in the Dashboard->Users section in case the person behind streathamswradio is unscrupulous.

What they have done is sort of identify theft.