If it's electric, ask them to provide you the law/regulation that states it has to be done by a "qualified engineer" (If they're installing electric hobs, they are very likely to actually be an engineer, but that is more a personal pet peeve that anyone who does something seems to be called an engineer, wheras they're probably more accurately described as a technician/installer, not an engineer). There is no such regulation (assuming it's domestic, I'm not sure of the commercial regs). The electrical regs only require installation by a qualified competent person where the work is notifiable, which, in short means installation of a CU, adding a new circuit, or work in a wet area (within a set distance of showers/baths, saunas etc).
If your hob was a replacement, and the appropriate electrical connection was already there, there's no legal requirement to get anyone in to do it. If the hob was damaged during installation, then they should prove this, and it would be the same as any other accidental damage (and if you got someone in to install it, you would then approach them to rectify, as they broke it).