Indeed it's topical - at the bottom are some of the notes for the students at my evening class last night...but meanwhile let's consider the case in question:
A could sue the originator of the libel, the publisher and the distributor.
That means he could sue Z, who blurted it out, the broadcster who let Z blurt it, the newspapers who reported Z's blurting and the distributors of those newspapers.
X is in the clear as she didn't say anything which, taken in iteself, could be construed as libellous (the phrase well-known TV presenter is too vague to imply any particular person).
Of course, A would only sue for libel if he was confident that the events alleged were untrue. In the event of a writ, the newspapers would be the first port of call, and they would presumably call upon A to make a statement, while any sbsequent police action against X as a result of claims now being made by B and C (eh? more bloody letters!) would have a bearing on the case.

Libel - Anything derogatory, critical, inaccurate published about a living person can be libellous. They can be identified by implication. Libel actions have to be launched within three years of publication. Repeating the libel can lead to fresh action.
Definitions:
A statement about a person is defamatory if it does any of the following:
Exposes the person to hatred, ridicule, or contempt;
Causes the person to be shunned or avoided;
Lowers the person in the estimation of right-thinking members of society generally;
Disparages the person in her/his business, trade or profession.
To succeed in a civil libel action, the plaintiff must prove that the statement complained of:
    Is defamatory;
    Has been reasonably understood to refer to him/her (not necessarily by name);
    Has been published to a third person.
Just as important is what the plaintiff does not need to prove:
    That the statement was false;
    That the defamation was intended;
    That the plaintiff has been defamed.
Someone could sue for libel even if the people to whom the statement was published knew it to be untrue. Only a tendency to discredit need be shown; the court assumes damage.
Worst Examples:
    Accusing people of crimes they have not committed.
    Alleging hypocrisy.
    Alleging they have contagious diseases, alleging they are so obnoxious nobody can work with them, alleging they're negligent in their work, alleging they are unemployable because they are so incompetent.
    Alleging dishonesty or immorality.
    Accusing them of sexual impropriety, alleging inaccurate sexuality.
    Accusing them of lying.
    Wrongly accusing people of doing disreputable things.
Danger words: adulterous, bankrupt, bribery, compulsive liar, communist, con, corrupt, coward, criminal, crook, drug addict, drug dealer, evil, fraud, fascist, Hitlerian, homosexual, hypocrite, immoral, incompetent, insane, insolvent, junkie, liar, Mafia, mentally diseased, misappropriated funds, Nazi, paranoid, pervert, pimp, plagiarist, lining the pockets, prostituting, quack, queer, racist, rapist, rip-off, satanic, scab, shyster, sleazebag, slut, spy, stupid, swindling, thieving, traitorous, unethical, unprofessional, unscrupulous, unsound…
Summarising the danger area of discussion:
Implying a loathsome disease; Falsely alleging arrest, indictment, confession or conviction of a crime; Alleging Anti-Semitism, or other imputation of religious, racist or ethnic intolerance; Suggesting connivance or association with criminals; Accusations of lying; Suggestion of involvement in racket or swindle; Claims of membership of an organisation which is disreputable; Imputing financial embarrassment or implication of insolvency or want of credit; Accusing of unwillingness or refusal to pay a debt; Suggesting poverty or squalor.
Defensive mechanisms:
It is important to be fair and accurate. There are some defences, however:
Justification
In essence, the words complained of are true in substance and in fact. It is a complete defence and also one of the most difficult.
Fair comment
Opinions expressed in good faith and without malice on a matter of public interest are safe - but read on.
The facts upon which the comment is based must be true. The only exception is where the comment is based on privileged material (see Privilege) such as a court report.
The defendant need not persuade the judge or jury to share the opinions which may be exaggerated, obstinate or prejudiced as long as they are honestly held.
Accord & Satisfaction
This defence to a libel action simply means that the matter has been otherwise dealt with. For example, the plaintiff has accepted the publication of a correction and apology.
Unintentional defamation
If the defamation was not foreseeable, as when a fictitious character is identifiable with a real person or where facts unknown to the writer make the statement defamatory. Under the 1952 Defamation Act there is a defence if: the words complained of were published innocently and an offer of amends is made in accordance with the Act.
Other defences
    That the plaintiff has died (dead people cannot be libelled or slandered in law);
    That the plaintiff agreed to the publication;
    That proceedings were not started within three years of publication (keep your notebooks);
    That the matter has already been adjudged.
It is possible to libel someone, even if you don't mention them by name. It's enough that they be identifiable from what you have written.
So, purely as an example, if one were to publish the claim that a former female Prime Minister of Great Britain was (insert libellous comment of your choice here) it would be no defence to say Margaret Thatcher's name did not appear. There is only one former female Prime Minister. That makes the identification clear.
Even more general references to less well-known people can be risky. "A certain left-handed accountant with a liking for golf..." (followed by some defamatory comment) could well result in a number of writs for libel, mostly from left-handed accountants you've never heard of, all claiming they could have been mistaken for the person referred to.
Privilege (Absolute and Qualified):- Something defamatory said in Parliament, Courts, Tribunals, Foreign Courts, and Parliaments, Public Meetings, Statements by Government Bodies can be protected as long as:
    You attribute.
    Your report is fair and accurate.
    And in most cases you give the person who is being attacked the chance to comment or contradict.
Absolute privilege attaches to reports from Parliament and the courts and when you publish in your earliest possible edition or broadcast. There are two kinds of qualified privilege. The first kind requires you to be fair and accurate in your reporting and relates to accounts of previous parliamentary hearings or court cases, or reports of trials and parliaments abroad.
Qualified privilege subject to explanation or contradiction relates to statements by government bodies, reports of public meetings, local authorities or associations. As a matter of professional practice it is sensible to take the initiative and solicit their response to the allegations. Then you could never be accused of not trying or failing to report their position.