I'm not sure, and unfortunately experience is that some phone agents deliberately agree deals they know they can't put through on the system, leaving the customer worse off, or having to fight to get the deal.
However, you're on absolutely solid ground because your legal rights are crystal clear - information provided by a company verbally or written is binding if the consumer relies upon it when entering a contract (Consumer Rights Act 2015), so VM have to honour whatever was offered and agreed, whether they like it or not. Normally in these situations the forum staff can check for you, and confirm the agreed deal has been actioned, or perhaps more likely not. If it hasn't they can get that corrected, and they'll be aware that a formal complaint escalated to the industry complaints scheme CISAS is very expensive for the company, and the company will be forced to comply.
