Ben - you might find this link quite useful...

http://www.moneysupermarket.com/community/forums/t/o2-and-default-on-credit-report-1305.aspx?PageIndex=7

In short ask for a signed as a certifiied true copy of the default notice which you are entitled to under consumer law. Also for VAT purposes T-mobile must keep all invoices with VAT added for a minimum of 6 years.

Don't be fobbed off and be persistent. Good luck!