I had a tenant who moved out but still owed me money. I contacted a local solicitor, who quoted me £2000 just to review the documents I had to supply (tenancy agreements, account records, bills, etc.) I decided to look for a more specialized firm. I had previously used Kearns, but they seem to have changed direction, so I found Landlord Law.
They were much more reasonable. They wrote a "solicitor's letter" advising him of my intention to take legal action, which did the trick.
It's easy enough to write these sorts of letter oneself, but when they come from a solicitor, they signal a much more serious intent to go to court. I think this is doubly the case when the firm specialises in this sort of action.
I have quite a lot of interaction with lawyers, unfortunately (no, they are usually delightful people, it's just that I need them when things have gone horribly wrong). Landlord Law take a more 'mass production' approach to a narrow area of law, which results in much cheaper services.