What can debt collectors do?

About 8 years ago I took out a loan for home improvements- but due to serious and genuine childcare issues had to pack up work. Unfortunately as a result I was unable to maintain payments and ultimately defaulted. This debt was then sold on to a debt collector. I contacted a debt counsellor who wrote to the debt collectors asking for the written agreement of the loan which they could not provide. I was then advised to stop paying this debt on the basis that because an agreemnt was not provided I was not liable. He also mentioned something about this being an illegal debt due to the period in which the loan was taken out but i am unsure on this. Unfortunately my debt advisor passed away last year- (he was working independantly through an ex forces welfare group) and as a result I am now getting harrassing letters and phonecalls up to 4/5 calls a day and 3/4 letters a week. The C.A.B cannot help me as I have credit cards which I am paying back every month. They have said that even tho this may be a statute debt (Im not 100% sure it is) they can only help when it comes to making offers of payment and that I have to treat all debts the same. This would mean (1) not only do I start paying a debt i was advised not to pay and (2) My payments to my crdit card company would be greatly reduced hence creating problems with them. I cant afford a solicitor at this stage however would be grateful if someone could give some advice as im now being threatened with home visits. My counsellor did advise at one stage that what the debt collectors had done is - buy the debt from the bank and through their own stupidity did not get a copy of the agreement. He advised that by implimenting bully boy tactics this is usually enough to get people to pay, however this is not legal practice. I guess a good boot in the groin when they do come could make me feel better but realistically not the best approach. This is genuinely starting to cause problems for me now. Serious answers would be much appreciated. Thank you