Tiscali are indeed a complete nightmare - signed up with them 6 weeks ago for their 8mb "max" product - then for the last four weeks have suffered from an unreliable service and speeds which were sometimes slower than dial up. I quickly realised that their customer service is a complete shambles so I got their HQ address from companies house and sent off a letter threatening a Small Claims action within five days unless they cancelled my contract - thankfully it seems to have worked as I am waiting to be connected tomorrow with a different supplier having got a MAC code.
If you find yourself banging your head against a brick wall with these companies the threat of small claims action is normally quite effective. Last year I had a nightmare with Orange who supplied a faulty handset and would not replace it - to cut a long story short I paid the court fee of £39, brought a small claims action at the local sheriff court, turned up on the day - Orange thought they could submit their defence by letter rather than appear and the sheriff granted a decree in my favour. Felt quite good sending Orange a letter demanding payment within 7 days or I would send in the sheriff officers! Got a cheque for £400 plus interest by return.