How do I find out account no. of closed mobile account?

dipsymoo

Hi, I don't know if I'm posting this in the right place but.... I have a default on my credit file from mobile provider 3G for a mobile broadband account I had.

I didn't pay the final months bill when I left them, so I had a default of £33 placed on my account May 2010, which as I had moved home at the same time, didn't catch up with me until March 2011, which I then paid.

As it's such a small amount, and having such an impact on my credit file, and will do for the next 4 years, I've decided to try and write them a "goodwill" letter asking if they'll remove it.

However, I don't know my account number, check my file only shows the last 4 digits, does anyone know how I could get this account number so that I could write them a letter?

Any help would be appreciated!
Thanks!