New flat immersion heater melting wires

[DP]

Just at the terminations at the timer and spur. I don't have it at hand but will count when I'm at home, thanks.

In that case may just be lack of tightness of termination. Cable overheat would heat the entire cable, it would be like someone had set fire to it

That would require that two electricians had not tightened then sufficiently.
The cable is known to be undersized. It is going to melt first inside the boxes where convection is limitted.

What is the immersion heater rated at?