One other thing that i did, is what you did, i went to a couple of neighbours, and tested their speeds, all 3 [and mine] were terribly slow.
When mentioned to tech support the reply was, "tell them to ring if they are unhappy", not "thanks, now we can see there is a problem".
So mentioning it probably wont help you any, and i think ntl rely on a lot of people not noticing their connection is duff.
It might mean having to fix something, if its fixable that is.