GB wrote:
Wiring immersion heater - how important is it to use special heat
resistant flex?

Also, will 1.5mm2 do for a length of just over 1m in open air, or is
2.5mm2 required?
Vital! The terminals on the element run hot by conduction and ordinary
PVC goes very limp and insulation could easily be displaced.
1.5mm should be adequate for the current but not sure what the regs
might have to say.