Finding Jarviser's website was the first ray of hope I came across, when I was first searching for help with broadband problems. It is full of useful bits of general broadband information that can help you overcome some very common problems, the sort of thing your ISP should tell you, but which they never do. It also has a wealth of BT-specific information that detail the idiosyncracies of BT equipment and how to overcome them.
It would hardly be exaggerating to suggest that Jarviser and his website have probably helped solve more broadband problems than the combined might of BT's first and second line support and he does it all in simple, understandable language, with good humour and on an entirely voluntary basis. If you can't find what you need on Jarviser's site, he provides links (some reproduced over on the left) to other sites which go into greater technical depth or are so fulsome it would be pointless to try and outdo them.
Every BT Broadband customer, in fact every UK broadband customer, should bookmark Jarviser's site. One day you might need the words of wisdom he provides.