Admission form sent then proceedings issued

[WATERHOUSE82]

lI have had a Claim for Service Charges issued against me. The solicitors for the Management Company say that they issued the Claim and have no knowledge of the signed Admission Letter which I sent to their offices. This letter was drawn by them and gives the authority for my lender to pay the outstanding charges. They are aware that by issuing proceedings they can claim their contractual costs that is stated on one of the provisions contained within my lease. I sent the Letter of authority to their business address and because I paid at my local Post Office counters by debit card I have payment proof supported by a letter from my bank to prove the transaction. I feel that they have done this to recover more costs [an extra £2000]. My understanding is that my letter is deemed served after two business days. Surely I cannot be held responsible for post that they are not receiving. The letter was sent on 10th November and their claimed served in January. Can someone help as I am powerless to do anything once the document was posted.