Re the slander, you'll need a letter from your solicitor warning them. If the things they are saying are true, then obviously the solicitor cannot help (assuming they are not though from the brief details you have given).
Re the court side and owed money, you'll need a copy of the contract and the chasers you have sent for the outstanding balance. Assuming they were late in paying and you sent a reminder then you just need to warn them of the pending court action by recorded delivery.
If still no payment and you are confident your contract and paperwork is water tight, then you can do a claim online - there is a fee payable for this. The other side then have to either choose to defend or advise the court they will settle.
If they defend, the judge can either make a decision based on the paperwork submitted by both sides or decide to so a small claims hearing. There is an allocation fee payable by yourself if it gets allocated to a hearing. If the other side fails to show you autoatically get awarded the judgement in your favour (providing they dont get the judge to agree a last minute adjournment), if they show you battle it out and the judges outcome is final.
You cant claim solicitors costs in small claims so unless you have free representation (dont know much about NCMA sorry) but can reclaim the court costs of issuing. The defendent gets the choice of court but assume you are both local to each other so this wont be an issue.
If you do get to the court issue stage, keep the details short and accurate on the form.