Plan B - check with UK cable makers/wholesalers, Prysimian, BATT, Nexans etc - if they make it see if they can help you move along the supply chain until you reach someone selling it by the m.
You could contact the people you bought it from and say that (yes you have consulted a qualified electrician) unless they can tell you where you can buy the cable the maker specifies you'll reject the hob as being not of merchantable quality.
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