huh - ok, I just saw this after buying the sugar...
I've got half Preserving sugar ('good for oranges, plums, blackcurrants, redcurrants, gooseberries, greengages, damsons and other fruits high in pectin' - so it says on the packet) and half Jam sugar ('good for strawberries, pears, peaches, raspberries, blackberries, blueberries, cherries, rhubarb and loganberries and other fruits low in pectin').
I got 6kg as that was as much as I had room for, but can always nip out for more tomorrow... working on the assumption we pick a kilo of fruit for jam making each, also that we need the same amount of sugar as fruit.
Various jam recipes call for 3:1 ratio of fruit to sugar, 2:1 or 1:1, so it seems a bit random!