I see the last post is quite old but it is quite relevant considering the financial situation recently. Furthermore would like to bring the topic back in the early-to-mid 2011 considering the need for advertising and the post financial stress situation.
We have tried Yell.com over the past 3 years and the results were amazingly low. I mean - NILL. No comeback at all. And now they have come up with their new service, sounds quite cheap: web-design + hosting + SEO for just over £170 a month. Considering the prices for standard SEO it sounds good. On the other hand 118 has proven to be with lower results than Yell.com, based on comments and feedback from colleagues.
My personal opinion is that similar websites do not work for domestic cleaning, cannot say anything about commercial and contract though. And even with the strong position search engines are currently taking I believe 118, BT directory, etc will keep loosing clients until they come up with an idea to beat the competitiveness of on-line search engines. Mind that there you pay for what you get, but not for flat existence on a list of companies.