If you say something like this:
Without Prejudice Save as to Costs
I refer to the above and to your client's claim for £355.20. You have added charges under the commercial debt regulations and my letter to your client has gone unanswered. I continue to dispute the debt and I await hearing from your client with their response to my letter in order to avoid litigation.
Your client is obligated to fully particularise their claim before they involve the court, in accordance with the Pre-Action Protocol for debt claims (“the Protocol”) contained within the Civil Procedure Rules (“CPR”). In particular, I refer you to paragraph 7 of the Protocol, and paragraphs 13 to 16 of the Practice Direction on Pre-Action Conduct and Protocols regarding the Court’s powers to impose sanctions for failing to comply with the provisions of the Protocol.
If your client does not respond to my correspondence, and goes on to issue proceedings, the same will be defended. I ask you to note that the court has discretion (in a small claim) under rule 27.14(2)(g) to award costs if a party "behaves unreasonably", examples being a failure to respond to letters/emails/calls, unwillingness to discuss the case and discuss settlement, a claim which has no merit, not following the court directions, etc.
I will therefore seek my costs if I successfully defend the claim against your client.
I look forward to hearing from you within 7 days with a substantive reply to my letter dated (insert date).