Air bricks are primarily for ventilation, to help prevent any damp issues etc.... Without being able to see it in an image or know its purpose it's a bit hard to tell what's its purpose may be but generally if it is there then it probably has a reason for being there so should be clear to ventilate.
In terms of a tradesman - your first stop should probably be a general builder as he should be able to tell its purpose and advise as to what to do, clearing it etc.... We have a find a tradesman service that may be able to assist you with finding one: http://www.diydoctor.org.uk/find-tradesmen/
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!