That didn't take much finding.
The legislation is at
http://www.opsi.gov.uk/si/si1998/98245102.htm
The relevant bit is Part B, Section 3, Paragraph 1.
Though paragraph 3 expands on this to say that no employee or self-employed person can (legally) do gas fitting unless the HSE say they are competent to do so. Which - I'm sure - will equate to being CORGI registered.
(You can also see that the word 'CORGI' appears nowhere in the legislation)
So you can do it yourself (if you're daft enough) without being CORGI, but no one doing it for you, for money, should be doing so without being CORGI.