Addressing people

Hello. I have a few questions concerning the word 'Mr.' and addressing people in England.

At the beginning of a letter, I think I should write:

Dear Mr. Brown, or

Dear Mr. John Brown,

in the middle of a text, for example :

'Please contact Mr. Smith in order to...' or

'Please contact Mr. Ben Smith in order to...'

When I am addressing the person I am writing to directly, in the middle of my text,

should I say :

Mister Brown, I would be grateful if you could...or

Mister John Brown, I would be grateful if you could...or

John Brown, I would be grateful if you could.. (let's say I have met the person a few times but it is not my friend, the letter is formal)

For example in TV studios, when an anchor addresses a guest, does he/she say:

Mister John Brown, thank you for coming.

John Brown, thank you for coming.

I would appreciate your help.