After reading all the post's on this thread so far I would say that due to the serious nature of this guy,s predicament unless you have have experience or knowledge then say nothing, many of the comments are flippant and will not solve the problem but create an opposite situation and when you are sitting in front of the magistrate in the small claims court trying to defend your case you will be taken to task by the defendant or his legal advisors.
I have been to small claims court several times and fortunately won each time, unfortunetly even though I have been awarded judement, on three occaisions I have not been paid because the defendants do not have any money or posessions or they have them and know how to work the system.
My advice for what it's worth would be to firstly satisfy yourself that if you take them to court and you win they have the funds or assets to pay you, items they have such as vans on lease or tools/plant on hire are worth nothing. You may be throwing good money after bad.
I have never used a solicitor and always defended myslef but I have spent a lot of time making sure I got things right and in this instance it looks like you have the law on your side. Having said that there are ways to go about things and this is expalined in the leaflets provided by the court, what I would say is read them, then read them again, then the following day read them again.
Look on the companies house web site and see what info you can find about them, how long have they been trading, is their reg office their trading address, what is their turnover, is it consistent with previous years.
How far are they away from you, go round one morning and see if the vans or trucks are going out the yard so you know if they are still working then go and see whoever gave you the job and have a word politely. Do not take anything or be agressive until you know they have no money or assets. Once you do this you can forget the small claims court.