Credit not being applied to account

I'm not sure if this is the best place to get this resolved, but I'm having trouble getting promised credit applied to my account without being brushed off by Virgin Customer Services.

 About two weeks before my contract was up for renewal I was offered (after quite a lot of hard selling by the Virgin accounts team calling me every day) a new broadband contract at £43 a month. As I had two weeks before my contract ended, I waited until nearer the renewal date to confirm I was accepting this offer. When the time came to actually arrange this contract I was told that the £43 price no longer applied, despite me having the contract documents already emailed to me. The new contract sent to me was now £57 a month. After a fair amount of back and forth and a lot of time on the webchat, I was told that the price of £43 a month could be matched, but would be applied as credit to the new contract.

Having signed this contract, after a few months it became clear that the credit was not being applied. I contacted Customer Services, and again after quite a lot of time and webchat exchanges, I was assured that the credit would be applied. Now having just received my latest bill, again the credit has not been applied. I've emailed Customer Services to raise this issue, only to be told that there's no evidence of me accepting the £43pm contract- despite me having the contract emailed to me, and transcripts of the subsequent webchats of the fiddling about to get the price back to what it should be.
I'd really like this resolved to the correct price, as in the current situation I definitely feel like I've been missold on the existing contract due to assurances on credit given to me. 

Again, I'd really rather not do this in the public forum but it does seem to be a much more efficient way of getting Virgin Media to actually do anything since Customer Services seem to be a bit ineffective.