Rinse & drain the blackberries in a colander in the sink

Put the berries into a heavy-based saucepan or preserving pan with 400ml/14 fl oz of water and cook on a low heat with a lid on for 20-25 minutes

Occasionally give them a mash to reduce them to pulp and squeeze as much juice out of them as possible

Still on a low heat, add the sugar and squeezed lemons (the entire lemon - seeds, juice and actual lemon!) to the pan and stir until all the sugar has dissolved completely (about 10-15 minutes)

Turn the heat right up and boil fairly rapidly for 8-10 minutes, stirring now and then to prevent it from sticking to the base of the pan

Carefully pour or ladle the blackberry mixture into a jelly strainer set over a large heat-proof bowl or saucepan (you could also use a square of muslin set into a strainer over the bowl/saucepan)

Using a wooden spoon, get all the liquid through as quickly as possible, squeezing the remaining pulp as much as you can - but do be quick, as the jelly sets if you take too long (if it does begin to set before you've had a chance to put it into the jar, just gently reheat it)

Decant the jelly into sterilised jars, cover with waxed discs and allow to cool before screwing the lids down tightly

The jars should store for a year or more