Hi andy
Unfortunately some lines do have SNR problems, and whilst there are quite a few things that you can do to improve your side of things - sometimes its just down to they way your line is :/
Things like good microfilters, NTE5 adls filtered faceplates (if you knew where the master socket is) and checking out your internal wiring can make a lot of difference.
At 55dB you may be able to get your ISP to call an engineer out to look at your line - there is something that they try to do for long length lines, and they may even fit a NTE5 adsl faceplate for you. Theres no guarantee on that - but it is worth a try.
The flip side of that is that if the engineer comes out and finds that things are as well as they can be - he may recommend that your line is unfit for 1Mb and reduce you to a 512kbps. This should improve your stability, but your line will be marked as only capable of 512kbps speeds.
>> Is it my Router, BT or my ISP causing the problem?
Its not your ISP, its not BT as such but it is down to your line conditions.
Some routers have proven themselves to cope with low SNR lines much better than others.
Something that may be worth a try is see if you know anyone that has a DG834x that you can borrow for a day or so, to see if that helps.
The D-Link DSL-G604T is another that seems to do ok as is the Speedtouch 536