This is not the full story. The regs do state if you are competent, from the Gas Regs...
PART B 3. (1) No Person shall carry out any work in relation to a gas fitting or gas storage vessel unless he is competent to do so.
BUT it goes on to say
PART B 3 (3) Without prejudice to the generality of paragraphs (1) and (2) above ... is a member of a class of persons approved for the time being by the health and saftey executive for the purposes of this paragraph.
To me, that's saying you have to be fully registered with gas safe (a way of proving you are in that class of persons approved) and have all other relevant qualifications (Part P, Part L, CENWAT, CPA1 [from April 2012 I believe?] (which ever ones you need for the job), etc) and have good practical experience.
I think you would have a hard time in court saying that you feel competent!
Just be careful you don't drop yourself in it & get a gas safe engineer in to do the work.
Thanks