Hydrolyzed lime powder
I suspect this is calcium hydroxide, also called slaked lime
I've seen in a small Asian grocery (in the US). It would also be available in Mexican sources as 'cal' (for treating corn for tortillas). I don't know what your options are if you are in the UK.
I wonder if it is still used to whitewash to cute thatched roof cottages.
Ah! Marvelous; I had been thinking lime in terms of the fruit rather than the compound.
It appears that you mix the powder with water, allow precipitate to form and then filter - only using the liquid, as the solids can be a tad nasty.
PS @ paulj, there are not as many of those as postcards and films would have you believe!