Thank you dx100uk

Hi, I'm new to this forum so please bear with me I part-exchanged my old car on 9th April for a 5 month old car which I got through a broker. I duly sent the old car's registration docment off to the DVLA along with the document for the new car. When I received a tax renewal doc for the old car I ignored it as it says on the back of the form if you have sold the vehicle and informed DVLA that these forms are printed 6 weeks before tax dosc or SORN runs out and I knew I had posted the relevant documents to them. In the meantime I received the new log book for my new car. Some three months later I receive a letter from DVLA for failture to tax my old vehicle. I replied on the back of the letter stating that I had returned the document for the old car along with the document for the new car and as I had received my log book, they must have received both as they were in the same envelope. Then received a letter thanking me for my reply but I was still to be held responsible for SORN or telling them that I had sold it so was still liable to pay the £80 fine. I finally managed to get through to the DVLA which was difficult in itself and explained the situation to a lady who told me to write again stating that I had put the documents in the same envelope so if they received the one for the new car they should have received the one for the old and that whilst the case is still be dealt with, if I'm still liable then could they uphold the reduced fee of £40. Never heard another word until 31st October I received my first of 4 letters from Rossendales Professional Debt Collectors. I have ignored each of their letters. The last letter received today says "Please be assured that this matter will not go away and we have every intention of collecting the outsanding balnce" I must contact their office within 7 days. This afternoon however, I had a call from a number I did not know and it was them. The guy said that he would be recording the call and could I confirm my name and address. I just said I will see you in court and hung up. Really not sure if what I'm doing is correct or not. Can someone please advise. Thank you.