Hello and welcome to CAG.
Firstly, EVERYTHING must be done in writing ONLY.
Never EVER discuss this over the phone with any of the fleecing DCA's.
If you're paying by DD then cancel it, and pay by standing order, so you control what you pay, this stops them from emptying your account when they realise you know your rights.
As this is a 'non-priority' debt you can reduce that amount, to a more realistic and manageable amount that YOU decide!
As for I&E forms, there is NO requirement to fill these in, and especially not one from a fleecing DCA, again YOU are in control, so YOU tell them what tune they dance too.
When did you take out the card with sharklaycard?
Pre or post 2007?
I'd be sending Link a CCA request, costs £1, if/when they fail to produce it in 12+2 (working) days you can stop payment until such time they do.
If you don't have all of the statements right from the start of the account opening, then you should send sharklaycard a SAR (£10) they then have 40 calender days in which to send you ALL of the details/statements from your account, which you can then go through and find all of the charges/fees and any PPI they have levied on the account and reclaim them, reducing the total amount owed.
Keep a diary of events, get all of the paperwork together, and NEVER discuss this over the phone.