Vodaphone complaint

Contacted Vodafone to discuss a SIM only upgrade. Was instead offered a really good deal on an iPhone 7+ which I accepted. Next day contacted them to see why I hadn’t received any order confirmation and they said order was cancelled (not by me!). Raised a complaint and they say their employee made an unauthorized offer (I suspect this was so he got a sales commission). Vodafone say they have dealt with him internally but are refusing to honour their employees offer to me, which I accepted. I believe they are legally obliged to do so. Anyone else experienced this with Vodafone?