I came across Mrs Whilla’s blackcurrant jam recipe in the recipe collection of Bettine Simpson.
It’s difficult to make out how many minutes Mrs Whilla boiled the fruit and water for, it’s possibly 20 minutes. I have said to simmer gently until the blackcurrants are tender – Blackcurrants have tough skins and must be boiled until the skins are thoroughly tender. If the sugar is added before the blackcurrants are cooked, it causes the skins to become tough and hard. The large quantity of water will reduce as the blackcurrants cook – most of the water needs to have evaporated before the sugar is added.