Thanks everyone for your suggestions. One call to Customer Services, described what I could get as a new customer, asked what they could do. They agreed to give me the same deal as a new customer (without the introductory discount) and arranged for an engineer to call the day after tomorrow. Result.
@carlwaring
I have no objection to new customers receiving an introductory discount, but I see no reason why they should get a better deal than people who have stuck with Cabletron / NTL / VM through thick and thin. It seems that VM now recognise this, so all is well.