If that promise was made face to face and there's no evidence, then chances are you've been had. If by phone there should be call recording evidence and they'll have to honour the offer.
So if this was a phone call, make a Subject Access Request to VM, asking for all data they hold on you. What you're really interested in is the call recordings, but ask for everything as a matter of principle.
If it was face to face then you can try Citizens Advice, and see what they have to say.