May be I am wrong...wolfy has proposed this formulation
Just an FYI, to make the equivalent of PBW, to the 70% Sodium Percarbonate 30% Sodium Metasilicate mix add:
Sodium laurilsulfate (detergent/surfacant, suggested mix 0.5-1%) is about $8/kg
EDTA (water softer, limescale remover, suggested mix 1-5%) is about $7/kg (only available in 25kg bags)