Send several letters stating your side of the case, asking for the matter to be resolved, and mention what (if any) response you have had. Include a reasonable time limit for a response - if it's a company, then 2 weeks is reasonable. Longer for individuals.
In the third or fourth letter - depending on how strongly you feel, and the amount involved, conclude the letter with your intent to take legal action.
If you end up taking action, then on the forms - online or paper - take time to fully document what has happened. Avoid aggressive language and don't be judgmental. Explain, clearly, fully and with perfect grammar, what loss you have suffered. If you aren't confident about your writing skills, then get someone to help. Explain the steps you have taken to try to resolve the issue, and the response, if any. Be realistic about your claim. You will be able to claim the court costs and the full value of your loss, but don't exaggerate it.
Turn up on the day and make sure you have ALL the docs with you. English county courts lose documents every day.
Be prepared to summarise your case. There's no need to be nervous, the judge will not be in robes or anything. Bear in mind it's likely that the judge will not have seen the docs or heard your case before he sees you.
Speak clearly and respond when asked questions. Don't interject.
If you win, make sure you get some agreed time frame for payment from the defendant.