Standard practice would be to fit a flex outlet place near the heater, run from there to the fused spur in 2.5mm twin and earth (you *could* get away with smaller, but its just not the 'done thing'), and then flex the heater up with 1.5mm heat resistant flex, 13A fuse in spur
Make sure this circuit is included in the sup bonding and note that the work you intend to dois notifiable