I had this same problem with mini credit/opos. Took out a loan of £200, and when it came around to paying it off I couldn't afford it.
I dug my head in the sand and refused to respond to any of their letters, and after a while my debt got transferred to opos, and was now standing at just over £1200!!
After a while of trying payments plans, I thought best to just ring Opos and see if I could agree to a settlement figure and clear the balance. Low and behold, they agreed! I paid £300.00 to clear my account in full, bonus!
Although everyone always says to contact by letter, sometimes just giving them a call is worth it!