I've had a word about your problem with Simon Davies, Clive, the MD at IDNet, and he's read through this thread.
While he'd be delighted to have your custom, he'd be reluctant to promise you that they could effect any improvements. Clearly, there appears to be a source of noise somewhere along your line and tracking it would be difficult. Having found it, a cure might be impossible. He told me of one case they had recently where the BT line ran over a water treatment plant. Which was fine until the water company decided to put in a new pump right under the phone lines. All the ADSL lines affected suffered a massive drop in speed due to electrical noise from the pump. The water company wouldn't move the pump and BT wouldn't move the cables.
Sadly, until BT are obliged to provide ADSL, this sort of thing will continue to happen.
