https://www.toolstation.com/shop/Electrical/d190/Cables+%26+Flexes/sd2730/Twin+%26+Earth+Cable+%286242Y%29+Grey+10m/p88694
Not suggesting you diy.
I've booked a professional. I lose interest when I have to start cutting through walls or playing with consumer units.
Sensible approach. Upgrading the mcb in the Shower consumer unit, or, can it handle the 51 Amp rated cable?
@hippy started
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https://www.toolstation.com/shop/Electrical/d190/Cables+%26+Flexes/sd2730/Twin+%26+Earth+Cable+%286242Y%29+Grey+10m/p88694
Not suggesting you diy.