Great job! Some of the red corrections are recommendations and not grammatical errors. They are just to make it more "native." (From an American perspective)

formal letter

Dear Sir / Madam,

I am writing this letter to complain about regarding a problem which concerns me. I have been living in a flat rented from you for last two years and, as I told you several times, the drains are blocked. Consequently, they smell really ("really" is kind of informal, I think) awful during the summer.

From my point of view, I believe that this smell might make my children ill. I would be very glad if you could call a plumber to solve this problem as the state of the flat is unacceptable. In the event that you cannot contact with a plumber, I can do it on my own. Once fixed the problem is fixed, I would send you the invoice.

This is going to be the last time that I will ask you to fix the drains. contact with you in a peaceful way. As you know, when I tried to contact with you last month, I received excuse upon excuse and I am starting to get very frustrated furious. ("furious" is very strong, I think)

Taking legal action would be my last option to choose. However, if I do not have any other choice, I will be committed to do it.

I am looking forward to hearing from you.

Yours faithfully, Sincerely ("Yours faithfully" sounds kind of weird)

Javier.