No interest to retain clients

We tried to do an upgrade over the weekend to an offer that was in your monthly book - valid from 1 May until the 31st of May. We we told that the offer had expired, we queried as to why it was still in your deals book and we were told that it was the marketing team's mistake. We then sent a mail to your retention department, who said they would raise with your marketing team as it was their fault - this was on Monday and we have not heard back. We also called on Saturday and spoke to a team leader, who also said it was a marketing department mistake. Should we just cancel the line, is nobody going to help us ? We want to upgrade to the 300GB smart data line as our 200 GB data allocation is just not sufficient anymore - if you can't assist us we will be cancelling, the false advertising is not acceptable though