I've never used the Robin Walker guide. On having a quick look it appears mostly ex-ntl, I'm clueless for the most part about that side. Acceptable ranges of power levels have changed at least twice in the last 3 years. When I started it was -4 to +4, now they tell us -12 to +12.
http://a.myby.co.uk/broadband-faq.html has loads of FAQ's and I know it's updated regularly.
HTH