Letter of Demand issued due to error on BCX end

BCX has just submitted a letter of Demand which is unrealistic. When I spoke to the BCX representative, I was told that it's for an outstanding amount. I could not understand why this happened as I have had a debit order since 2015 for the collection of subscriptions. I was told that there was a billing error on the BCX side.

I was never made aware of an outstanding amount, nor was I engaged to settle an old account, that I was not aware of

The first interaction I receives on this matter was this Letter of Demand