This is both very interesting, annoying and a pain to use - all at the same time!
If you call UK Number 0800 118 3733 you will hear a few adverts from sponsors an invitation to sign up for the service (which you can ignore) and then you are asked to speak the name of the party you are looking to find the number for, then the location and then some more advert and then some music and then ... you get your number. My test call worked just fine. It could be useful for those like me who are outside the UK who can't dial into a normal DQ service or those who wish to avoid the heavy cost.
If you are near a PC then it's still much easier to use the online services at
http://www.192.com/ or
www.bt.com/directory-enquiries