From an impartial position the solicitor has not inferred that there is evidence, they have merely stated that you may be charged if the police to find any evidence.
Who is the solicitor acting for, you or cps?
You may have a complaint against the police for wrongful arrest, if you have evidence to back it up.
From an objective point of view you have been told that you are not being charged due to lack of evidence but could be if further evidence is discovered.
You know what you have or have not done. The solicitor does not know with absolute certainty your actions and whether new evidence may be discovered. the solicitor is merely putting you on notice.
You have said that you did not commit any offence but only you know that with absolute certainty.
Now if you think, this letter or the solicitor's actions constitute a breach of the solicitors' code of conduct then you can call the legal services ombudsman and/or the Solicitors Regulation Authority.
What is your complaint exactly?