RCS - Not treating me as a valued customer

I asked for a settlement letter on the 28.12.2017 and received it the next day 29.12.2017 on the letter was the notification that the settlement amount is valid for 5 working day. I paid the amount stated on the letter within the stipulated 5 working days period. On my bank statement it’s stated that the amount was transferred on the 2nd of Jan 2018 from my account to the RCS account, however I’m now battling to receive a letter from RCS stating full payment receipt and zero balance owing on the account. RCS now claims that I didn’t make payment within the 5 working days period noted on the letter. I called the contact centre and got sent from pillar to post and still no sound feedback, I was then given a team leader’s email and contact number, however the team leader/supervisor is never reachable and doesn’t respond to any of my emails. RCS is not abiding by the TCF principle, more so they’re causing me unnecessary stress and now my ITC record hasn’t been updated with a positive record that I handle my financial affairs in a clear-cut way.