I just found the official document I thought I remembered and I was correct, the relevent section reads,
"If you have not obtained a CNI issued by the local Registrar of Marriages in the United Kingdom you will have to comply with the Marriage with Foreigners Act 1906. This requires that you reside in the Philippines for 21 clear days before giving notice of your intended marriage and this period of residence must be immediately prior to acceptance of the Notice. Twenty-one clear days means that the day you arrive in the Philippines or the day you submit your Notice of Intention to Marry may not be counted. When the notice is accepted, your physical presence in the Philippines is not required.
When you give notice of your intended marriage, you will be required to swear an Affidavit before a Consular Officer to the effect that you are not aware of any impediment to the marriage (including previous marriages). Your Notice of Marriage will then be displayed in a public place in the Embassy for a further 21 clear days (excluding the day you swear the Affidavit). The CNI will be issued the day after the 21 clear day period has elapsed and you should collect it from the Embassy as soon as possible thereafter. The CNI must be used within 3 months of date of issue. This whole process therefore takes 42 DAYS to process."
You can download the complete document here,
http://ukinthephilippines.fco.gov.uk/resources/en/word/7164698/21633626/17275017/cni-guide-sep-2009
Iain.
