Advice re: Vodafone/1st Credit collection

Is it legal for a company, lets call them 'Vodafone', to sell the answers to your security questions to a third party ?

I have been contacted by a company called 1st Credit claiming that I owe £67 for an outstanding sum I owe to Vodafone from 2004 !

I didn't have any idea who they were and they started asking for my Mother's maiden name, to which I refused to answer.

This has been going on for a couple of months now... It's not a lot of money but its the principle. I have repeatedly asked for invoices relating to this sum, to which they never respond to, they just continue to ring me and send me letters.

Last week I rang them and told them if they cannot prove the sum, they should either issue the summons or leave me alone.

They gave me the telephone number for Vodafone and my old account number and told me to get the invoices myself !!

I rang Vodafone and they have e-mailed me three invoices amounting to £51, but they wrote this 'debt' off in 2004 and have obviously sold it on with all my security details . I am astounded if this is legal

Am I within my rights to continue to insist that 1st Credit supply me with proof of debt even though I have obtained some details myself (I have no idea what the additional £16 is for) ?

I need to contact my bank to get copies of my statements for the latter half of 2004, and I am also goingto get Vodafone to provide me with a copy of my letter terminating the contract... in the meantime, I have these !!!!!! ringing me every day telling me to pay...

I am not trying to avoid paying, if I owe it , I owe it , but I am determined to get 1st Credit to send me some documentation before I cough up anything.