Complain, log the complaints you make as well as the replies or non replies, etc etc. I think you have to have shown to make a reasonable effort to 'fix' the problem. This means you have to go through whatever their support tells you.
Thats not really fair I know, especially as its probably them and getting hold of them is difficult, but thats just the way it goes Im afraid.
To be honest, your probably going to get it done quicker by simply waiting for the last 2 months of your contract to be up.
And when you cancel, make sure you stop the payment (once the last one has been taken) as Tiscali have a long record of continuing to take payments.
Make sure you leave as many reviews as possible (Here - thinkbroadband.com, and any others you can think of).