If you have not signed off on the add, (ie they changed the add that you signed off on then they are in the wrong and the add is null and void), debt collectors are not allowed to enter your premises, and you say this to them on the phone as well as at your door step with out a court order, if they do they are trespassing, and all you need to do is call 999 and say intruder, that's all and the plods will be over in a tick.
It is against the law. I have been through this before.
They will usually take the tack of do you want to discuss it for all your neighbours to hear or do you want to talk inside.
Tell them to go away and off your property, as soon as they are inside, they start to make notes of what you have.
YP, take about 18-24 months to follow with court action, if they even do, most of it is talk unless it is over a grand. They just put it to the debt monkeys, so let them take you to court, if you didnt sign it off they haven't got a leg to stand on, and because they have offered you a £200 reduction, that is them admitting to guilt, do not accept the £200, tell them that you want the whole thing refunded, and you are now talking to your solicitors about taking them to court for loss of business, as they screwed up on the add, however you need to prove that the business is not coming in from that add, ie if they have put your telephone number in wrong, you will get no business, therefore loss would be X.
Hope this helps, if you need any more let me know
Aidan