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.
This is the catch-all and would seem to apply to DIY work; though I have never heard of any DIyer being prosecuted for breaking this regulation.
Oh yes there is a case
Read Rhondda Landlord Prosecuted
He was done for breaking both Reg 3(1) and Reg 36(4).