Which is best for laundry & other cleaning please – stabilised vs unstabilised sodium percarbonate?

I didn’t actually know about the stabilised form of this until I saw your question. It’s an oxidiser (so it works by oxidising stains) and like most oxidisers it’s unstable, as left to its own devices it will fine something to oxidise, and so expend itself. So the stabilised form will be the better of the two – but it just depends on the relative costs. If you could get the unstabilised form cheap, it’s efficacy could be preserved if it was in a sealed container where the atmosphere couldn’t get to it.