Preheat your oven to 150°C. Place your sugar in an ovenproof bowl or pan in the oven for 10-15 minutes, and remove it before it starts to caramelise or to cook around the edges. (Heating the sugar in the oven first is a tip I read from the fabulous Darina Allen, who recommends doing this to help keep the fruit fresher, as you don’t have to boil the mixture for as long in order to dissolve the sugar.)

Place all your ingredients apart from the sugar in a heavy bottomed saucepan or preserving pan. Bring the mixture to the boil, then lower the heat under the blackberries and simmer steadily for 15 minutes or so, until it is soft and the blackberries have broken down. If any of the blackberries remain whole, use a heat-proof silicone jam spoon or long handled wooden spoon to mash them up as you stir.

Add the sugar and stir until it has totally dissolved. Bring the mixture swiftly to a rolling boil, stirring in one direction to help prevent sediment forming on the top. The mixture must reach a temperature of 104.5°C in order to set – use a jam thermometer to check this before decanting the jam into the jars.

If any scum has formed on the top, simply skim this off before pouring the jam into the sterilised jars. To seal the jars, immediately place a wax paper disc on the top of the jam, with the wax side facing down. This will help prevent any mould from forming. Next, make the jars airtight by placing a cellophane jam pot cover over a damp tea towel or clean dishcloth to wet it slightly. Place these tightly over the top of the jar, and attach them firmly using elastic bands. As the cellophane dries, it will tighten and stretch over the jar, helping to keep the jam airtight.

Tip – if you don’t have a thermometer, to test whether your jam is ready, chill a plate in the freezer in advance. Once you think your jam has reached the right consistency, drop a little jam onto the plate and drag it along with your finger. If it creases up with a jam-like consistency it is ready, if not, return the mixture for a couple more minutes to the heat, before testing again.