ash out the apple pan and return the apple juice to it, measuring as you do so.
There should be around 1.5 litres. For every 250ml, measure out 140g granulated sugar (ie 840g sugar for 1.5 litres liquid) and stir it into the apple juice.
Heat through gently, stirring until the sugar dissolves, then bring to the boil and simmer briskly for 15-20 minutes, skimming any scum away, until the jelly reaches set point (105°C/220°F on a sugar thermometer or the point where slow double drips - rather than one quick drip - fall from the stirring spoon).
Let cool for a few minutes.
Stir in the rose petals and their liquid with the remaining 100g granulated sugar and the lemon juice.