Hi and

to CF fairtrader ...
NTL are extremely unlikely to pricematch, as from the "big picture" view, one subscriber is pretty small fry.
Don't forget that if you downgrade to 600, it will eventually (when they rollout the upgrade) be 750 - for £25 - not a bad price.
Also (eventually.....) the 1M will be 1.5M for £38 - again, not too bad a price..
I would suggest that you do a comparison based on these speeds if you are thinking of changing supplier - simply because you are looking at the cost/savings over the course of the year, and the speeds
should 
be implemented by "late summer".
If you then decide that BT/ADSL is the way to go - go for it, NTL will not fight for one user, as they have quite a high number of new subscribers coming online ATM..
One day, the number of new subscribers will be less that the number of leavers, at that point NTL will have to start looking at the problem, until then, they are unlikely to do so, as their income from BB will continue to grow.
Just my 2c as always...
(oh, and I cannot get BT BB where I am

so I am pretty much stuck with NTL anyway - roll on 1.5M

)