get a bottle of high phos from evans vanodine (its toilet cleaner)
apply to the scale
leave ten mins
aggitate and leave for a bit longer
scrub it of with a green pad, should come off fine, i`ve never known Hi-Phos to do any damage to plastic.
wear gloves and mind your eyes cos it`s evil

careful with it if you use it on stainless steel, make sure you rinse it all off or it goes green and eats into the metal :
