There are so many locksmith directories out there now that it's difficult to know which ones are worth adding your locksmith business to.
Well the answer is simple really: As many free listings as you can.
If a site is insisting you pay to list your locksmith business the general rule of thumb would suggest that it's not worth investing in. If they are really as great as the claim then they would surely offer a free trial to prove so.
There are a couple of directories aimed at promoting the small local locksmiths that I would recommend that are entirely free. They do offer a paid feature but it's not mandatory like most of the paid locksmith directories.
Firstly: http://www.locksmith-directory.org.uk is the original 'No nationals' directory that are lot of people are now trying to re-create. This directory ranks no.1 for the term locksmith directory so is a good way to be found online for customers searching for a directory. All entries are vetted and checked so there are no national and spam firms listed here.
Second one worth mentioning is http://www.doors-repaired.co.uk a similar directory built on the back of a locksmiths site that lists all genuine local door repair firms in the UK. Again the same No National locksmiths ethics apply.
A lot of the magazine publications are also creating online local locksmith directories that you may want to add to, just be warned that they will cold call you and try to get you to subscribe to the magazines in return (which we all get free anyway).
Then there are also a lot of seo and web companies with similar themed locksmith directories all looking for paid entries and making a lot of promises. In my experience these are best avoided if giving the hard sell on the phone. This includes Yell.com and Thomson local who both offer free listings but will pester you to death with cold calls afterwards.