OK, so they're definitely all in kilobits per second?
In which case... your upload is awful
The "750k" service (750 kbps....kilobit per second) should have an upload of 128kbps (128 kilobits per second).
...Yet you appear to be getting only 16 kbps (16 kilobits per sec)!
Try this upload speed tester:
http://www.pcpitstop.com/internet/BandwidthUpTest.asp
I'm on the 1.5meg service (with a 256kbit/sec upload), & just got
(where "Kb/s" = kilobits per second)
...Which is pretty good.
If your upload really is as bad as it seems from your earlier test, then it could be that your local UBR is oversubscribed.
Try phoning ntl again, & explain to them about your upload being so unbelievably poor.
Also, hopefully one of the ntl people here will see this thread & try to give you some help.
If it does definitely seem that the upload is the cause of your problem, I may well move this thread (& add a bit to the title) to the Broadband support forum - could get a better response there (& it would be the correct forum, if the problem is the upload, & not simply email problems).