http://www.ofcom.org.uk/
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Sounds a bit boring I know, but they've done quite well on one or two ISP related issues lately.
[See: 'Complain' icon link]
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In addition, BT Wholesale should be able to confirm to you who has a tag on the line (as obviously
Talk Talk need convincing!). See
here, para 4:
0800 169 0934 (maybe able to give you some general advice also on the situation?).