Thanks for your patience.
Libel and slander are both forms of defamation.
Many people are keen on suing for defamation without having full appreciation of the law or practicalities in doing so. I will try and clarify the position below.
First of all, certain conditions must be met for the statement to be classified as defamatory. These are:
1. There has to be a defamatory statement - the assessment that is often used to establish this is whether the statement tends to lower the claimant in the estimation of right-thinking members of society generally
2. Its publication has caused or is likely to cause serious harm to the reputation of the claimant – this will vary based on what effect it will have but it really has to be something sufficiently serious
3. The statement has been published by the defendant to a third party
4. The claimant must prove that the words complained of were published about him - this should not be an issue if the claimant is named or clearly identified.
Whilst it may be easy to prove that defamation has taken place, the legal process of pursuing such a claim is often complex and prohibitively expensive. A claim must be made in the High Court and will likely require the help of professional defamation lawyers, so the costs will often be high right from the outset, usually in the thousands. There is also no legal aid available for such claims so the complainant must fund these personally.
In your case, the fact that this statement is made within the context of family law proceedings to Cafcass (a professional body) also causes difficulty. Family law proceedings are private proceedings, they are not conducted in the public sphere. In the context of family law cases, there is normally one party saying untrue things about another - it is the nature of the area of law. These comments weren't therefore in the public sphere and it is going to be nigh impossible to prove any reputational damage that could lead to a successful claim on the basis of information used in the context of court proceedings.
However, information shared publicly is another matter.
Instead of starting legal proceedings it would be best to contact the publisher of the comment, advise them that what they have done amounts to defamation and that you will consider pursuing the matter further if they do not retract their statement and issue an apology. This could prompt them to reconsider their position, which would avoid the need for court action. Of course, if they refuse to comply the option of suing still exists, but consider the above information before going down that route.
I trust that this assists?