Cicero coughed up some electrons that declared:

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I'm a bit confused by the OP's post. Can you actually buy three core cable
with the earth wire separately insulated in the same way as the other two
cores are insulated? I understood Fred's post to indicate that the earth
is always a bare conductor in twin and earth cable whereas the OP's post
suggests that his cable includes a fully insulated green/yellow earth.

It's probably flex, based on the fact that heating is involved. I suspect it
the flex going up to either the hall stat which probably wants E, L,
Switched-L *and* neutral for the bias heater.

Or it could be the flex going to the programmer and the bloke only had 4
core, when he needed 5 or whatever.

Cheers

Tim