Sound advice from Sharpley, and I would add that the first time you receive a notification from an agency you write back to them instructing them to freeze any interest on the debt whilst you are in dispute. If the debt is still with the originator and they have instructed a DCA to act on their behalf then deal with them directly and not the DCA. Write or e-mail (I tend to write and send important letters recorded delivery as e-mails can get deleted) when corresponding with them, and again, if they have requested a DCA to act on their behalf, request that they suspend activities whilst you resolve this with the originator. Only deal with the DCA directly if the debt has been sold on, and then again follow the steps above.
You can make a fair offer to repay interest free via instalments. If they reject the offer then visit your local Citizens Advice, and have them work out a pro-rata payment for all creditors against disposable income, which can often work out a lot less than what you offered in the first place. Like Sharpley, I'm speaking from experience