The website you've quoted from (which is here, for anyone else with an interest in this thread
https://www.internetlawcentre.co.uk/legal-liability-of-review-website-operators-in-the-uk ) has all the main points covered correctly.
Information about what constitutes a formal notice of complaint can be found here:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/269138/defamation-guidance.pdf
Some practicalities need to be noted here though:
Firstly, for an action for defamation to succeed in the courts, there is a need for the claimant to prove that it has caused, or is likely to cause,
serious harm to his/her reputation. [Section 1(1), Defamation Act 2013]. A statement might be totally untrue and utterly malicious but, as far as the law is concerned, it's not automatically defamatory. The 'serious harm' test, which was introduced by the 2013 Act, is quite a stringent one and many attempts to prove defamation, which might previously have succeeded, are now likely to fall at that hurdle.
Secondly, there's no legal aid for defamation cases. The reality is that it's impossible to sue for defamation unless you've got (and are prepared to risk) at least a five-figure sum in your bank account. (The only exception here might be when someone else is prepared to take a case on, on behalf of the claimant. An example might be a teaching union who're prepared to back an action by one of their members against a parent who has made a defamatory statement which could prove detrimental to that teacher's career).