So the short story is that 1 year ago I called Crooked Communications Company to cancel my phone contract and was put through to retentions. I was paying £14.85 a month at that time and they offered me £16.20 a month to stay with them, saying that was the lowest they could go. I accepted as it was still better than I could get elsewhere.
Fast forward a couple of months and the bills were coming through at £24.30. On querying it I was told that apparently the notes didn't support what I was offered and instead say that I'd accepted £24.30 (a 10% discount). 
But as I had been paying £14.85 the previous year (a 45% discount), they saw that it was reasonable that I'd have been offered £16.20 (a 40% discount) to renew. I was promised a credit for the difference for the whole year as they couldn't amend the monthly amount. This never materialised and still hasn't. It transpires that there are no notes for this conversation either. 
I've raised a complaint with CCC and we have reached a deadlock as they won't give me the credit. This has been going on for months.
I have now been told (after 1hr on the phone they finally decided to mention it) that they have obtained a recording of last year's phone call in which I had upgraded and the transcript says I was offered a 10% discount and I accepted it. They are blatantly lying as I know that's not what happened. I told him that they should be ashamed to have sunk that low and tell me such a barefaced lie.
AIBU to think that the original retentions guy had a target to meet and offered me 40% but put through 10% as he might not have been supposed to give me 40%?
I think it's suspicious that there is no record of that conversation (though it does state that I called to cancel) or the one where I was offered a credit (even though my phone records show I had a 35 mins call to 191).
And they are now being completely underhand in pretending they have a phone recording that disproves what I'm saying, even though I know I am right. He even said he had read the transcript himself and they would make the recording available to the Ombudsman on request, as he dug himself deeper trying to maintain that he was telling the truth. 