Just for comparison (and to show this is not just an NTL problem).
I have just called Vodafone, and asked about upgrading my phone, and changing tariff.
I was told I would have to pay to upgrade my phone to a V550, and the best tariff they could offer to fit my needs was £22 a month.
I went to their web site and found the same phone, with a free bluetooth headset, on the equivalent tarrif for free, and £18 per month.
When I pointed this out, I was told that the upgrades department don't deal with accessories, and when I probed further I was asked if I wished to speak the their retention people - which I did.
The best they could offer was £50 for the phone, no accessories (we dont deal with accessories) and a discounted £15 per month for the line rental for 6 months. (not including the free weekend calls currently available).
When I pointed out that this would cost me more that the deal they were offering for new customers, I was politely told that this was true, and that this is because I have a spend history with them, and the discounts are based on this. A new customer has no spend history, and is therefore given the highest discount available.
The person I spoke to was polite, but basically summarised my options as "like it or lump it". I chose to lump it, and have ordered a new phone, on a new contract, for free, with the bluetooth headset for free, and the free weekend calls, all from their own website - the only difference is that I will have a new mobile number (their deal was better than O2 or Orange, otherwise I would have moved and kept the number ).
This just shows (and the guy I spoke to at Vodafone agreed) that new customers are treated better than existing ones by a large number (I would say the majority) of the large corporates.
It does seem that this is an attitude of - we get our money out of you in the first 12 months - anything after that is a bonus.