Anyone installing NEW 'fixed electrical installations' must ensure that their safety is checked by a suitably qualified person. (A qualified electrician can check his own work. If you, or another unqualified person, puts the installation in - as you're perfectly entitled to do - you must then get the work checked by a qualified person).
However the rules don't cover repairs or replacements to EXISTING fixed installations. For example, anyone can replace a mains socket, light switch or ceiling rose without the need for any checks. If you already had the wiring going to a hob, and the fitters simply replaced the hob (rather than installing an extra power circuit to supply your new hob) it's likely that their actions would be regarded as akin to changing a socket, and therefore not covered by the legislation.
If one of your rings is permanently on (except when the power is turned off at the wall) it seems most likely that there's a fault with the rotary switch within the hob. (i.e. the fault is not the result of poor workmanship by the fitters). With most hobs it would be extremely difficult to wire power to the wrong side just one switch, while leaving the others wired correctly.
Chris