Mat you could run a tracert to try and see if theres a point that it slows down, just to see if its at your end, BT/NTL or further on.

I did one once when I had a slow connection and it ended up that it was at the exchange (they were installing the new Digital subscriber network) so some lines were multiplexed up quite high and hence giving a slow connection. But when I tracerted it it showed that there wasnt a porblem on my side as it went from my pc, to my router and then onto the network, slowed down at the next couple of hops, then speeded up.