Low line speed

I have a Belkin modem/router and a Belkinwireless adapter, but I really don't think these are my problem as I had appalling line speed BEFORE I went wireless. In fact, I went wireless BECAUSE I had poor line speed and wanted to rule out the cabling in my house.
The last week or two have seen my line speed fall through the floorboards, at one point getting as low as 7Kbps!
I have run several different Broadband optimisers and wonder if this is where my problem really lies?
My ISP is Tiscali who recently LLU my exchange and upgraded my connection from 512Kbps to 1.1Mbps. They are adamant that my line speed should be showing in excess of 560Kbps even when demand is heavy and that my line and their equipment are not the problem.
My connection occasionally hits in excess of 570Kbps but doesn't seem to want to stay that high and is still considerably less than I would expect so if anyone can suggest what my settings ought to be and what details you need in order to determine this I would be most grateful.
Seriously frustrated just doesn't come close to describing how I feel at the moment.

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My ISP is Tiscali

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There's your answer. Just spread the word so nobody else makes the mistake of using them and get out as fast as possible.

Not wishing to argue with you but I suspect (as poor as they are) Tiscali aren't the problem. My next door neighbour (same exchange and line) gets good speeds most of the time.
It just seems to be me having trouble.

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My next door neighbour (same exchange and line) gets good speeds most of the time.

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Which ISP ?

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Tiscali aren't the problem. My next door neighbour (same exchange and line) gets good speeds most of the time.

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As do I, while my next door neighbour, on Tiscali, is stuck on a 512k connection...

In the unlikely event that they too are Tiscali and you can see direct evidence (not heresay) that they get decent speeds while you get poor ones then the things to question are...

how are you measuring your speed?

do you see large numbers of errors reported in the router stats?

Have you tried testing at the NTE with all extension wiring disconnected (as shown in the link) ? Many poor speed faults are due to customers own set up and equipment, that said Tiscali are a strong contender for a root cause also.