This largely depends on how much money you are owed? If it is less than �5,000 then the procedure for taking such a claim through courts is fairly simple and you would not require the services of a solicitor. However you will need to think carefully about whether you have sufficient proof (a written and preferably signed contract?) ans whether the defendant has a counter claim - was your work up to the required standard??
If the answer is yes to all the above then you can quite legitimately get aggressive. Send a clearly worded but dispassionate letter spelling out your complaint and requiring them to pay. You can then ultimately follow the court procedure and make a civil claim.
However, if you have no written contract or the amount owed exceeds the threshold then you would want to be a lot more diplomatic in your appraoch. Try speaking to them and maybe be prepared to negotiate a bit on the amount of your bill.