Home DIY

Doesn't the actual legislation just state that you have to be competent? If it's DIY, that is.

Exactly. The regulations don't apply to DIY, just the legislation.

The Gas Safety (Installation and Use) Regulations 1998

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

Then

Part B 3 (3) Without prejudice to the generality of paragraphs (1) and (2) above and subject to
paragraph (4) below, no employer shall allow any of his employees to carry out any work in relation to a gas fitting or service pipework and no self-employed person shall carry out any such work, unless the employer or self-employed person, as the case may be, is a member of a class of persons approved for the time being by the Health and Safety Executive for the purposes of this paragraph.

When you are at home working on your own stuff you aren't employed of self-employed so B 3 (3) doesn't apply.

It's arguably bonkers but it is what it is.