Do you have proof of posting?
Should any debt collector call tell them to get lost and to obtain a court order, then go to court and explain the situation the judge, if it gets that far. Should you loose which I hope you don't, the court will take your circumstances into consideration and may say that you can pay it off at 50p or so a week/month.
Under no circumstances let any debt collector into your home and do not be intimidated by them, they have no powers to enter your property, only a court appointed bailiff armed with a court order can do that, stand your ground, be firm and polite at all times. If they persist report them to trading standards and if necessary, call the police.
Hopefully it won't come to that though.
LB