Well, time to update this thread... the NTL engineer arrived today (earlier than scheduled too
).
Had one look at my connection with his kit, said I needed an attenuator and promptly installed one. He also kindly swapped out my Ambit 200 for a nice, new 250 just cause it's a better model
My connection is now completely back to normal... just downloaded a 650MB file from Giganews in about 12 minutes
Took the engineer a total of about 7 minutes to resolve my issues, and 2 of that was him moving his van as he'd parked in my neighbour's space. He was a nice bloke as well - even explained to my nipper what he was doing and how it all worked.
So, respect to the engineer
Just a shame the Indian call centre insisted for so long on trying to resolve things via their scripted responses instead of getting this guy on the case straight away.