Sorry for the late reply; exams and all that now
Getting the growth and barnacles off is fairly easy; getting the sponsons looking decent is (I've now conceded) impossible.
I've found the best way of getting them off was to spray them down with the HG Mould Spray and agitate, wait a few minutes then wash off thoroughly with water... waiting a week or so and then knocking them off is easy with something blunt and plastic.
Cilit Bang was decent for getting the black marks out from between the barnacle feet marks but didn't make much of a difference to the feets' rings themselves.
PolyMarine Inflatable Cleaner is awesome for getting rid of the oxidation and grime but, again, doesn't make any difference on the barnacle rings.
I've got a before and midway photo attached; when I'm next down at the boat (next week after the the transom gets cut out and replaced; rotten wood
) I'll get an 'after' photo too...