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Thats exactly where it becomes murky if something goes wrong and then people are looking for someone to blame. Which is very easy if you dont really know what your doing but the can be said for a lot of stuff.
Id debate what competent is really, IMO its someone who at least got the qualification even if you aren't on the gas safe register but then again you cant do work unless you are on it and also need to renew every 5 years to be on it.
Lets face it for the price of £80 quid is it worth even risking it.
The law says if you're doing gas work for money, you have to be registered (ie on the Gas Safe register), but if you're doing it for yourself or as a favour you have to be 'competent.'
Competence is carefully undefined, though it's a fairly safe bet that is something goes bang then someone official will ask questions.
I'd change a cooker on a bayonet fitting for myself without too much hesitation (though I'd be wanting to sort out a manometer, even though the fitting is designed to self seal): I'd think a couple of times before doing a hob or anything more involved.