Urman wrote

I am in the UK and have an old portable electric heater.

The mains flex to the heater is like that to a domestic iron:
woven fabric outer overlaying a rubber (?) outer sheath.
With each of the three leads covered in rubber insulation
and three cords running alongside the three leads.

(1) Is the fabric outer to make the flex resistent to KINKING?

Nope, its just there to keep the 3 inner wires together.

Its a bit more flexible than the more common plastic or rubber outer.

Or is it there for HEAT PROTECTION?

NOpe.

(2) Is the RUBBER used in this sort of flex
especially resistent to TEMPERATURE?

Nope, its what was used before plastic was cheaper.

(3) Are the alternative materials (such as silicone rubber)
used in modern equivalents to this type of lead SUPERIOR
IN ALL RESPECTS to the sort of flex I have described?

Not really, its just cheaper now than rubber.