Barclaycard,Monument & Link financial

In the last few days we received a letter from Link Financial (debt collection) introducing themselves informing us that they had taken over an old monument card debt owed by us. Nothing unusal there you might think. On reading the letter it stated they have had the account since 2006, almost three years and have not received any payments from us.
We have been in an unofficial payment agreement with Monument all this time and been sending them our monthly payments without fail as well as successfully reclaiming all the ridiculous charges and interest charges they piled on to a small debt to make it a large one.
We tried to contact monument (almost impossible as they dont have a number) to be told they were now part of barclaycard.
We rang Barclaycard but they have no record of the account whatsoever. The debt has been reduced to a few hundred pounds but Link Financial want £3900.00 !!!
They cant tell us anything Barclaycard don't know what day of the week it is never mind where all our money is and Monument dont have anything either.
The account as you can imagine has been closed and past to the debt company but we have no way of finding out the true balance . The only possible method I can think of is to put a statutory request through to Monument demanding the charges over the last six years and hope it shows all the transactions. We havent had statements for the card in three years.
Does anyone have any advice on how do deal with intellectually challenged at Barclaycard and Monument??