Halve and stone the plums. Place in a large pan with the sugar and 3 tablespoons water, cover and cook over a medium heat for 10 minutes, stirring occasionally, until the plums are releasing lots of juice. Then simmer over a low heat, covered, for 20 minutes until the plums have collapsed.
Transfer to a jelly bag or 2 sieves and leave to drip for about 2 hours (don’t force the syrup through or the jelly will be cloudy). Measure the plum syrup – you should have about 750ml-1 litre plum syrup. You need 1.1 litres in total, so stir in a little cold water to make up this amount. Discard or reserve the cooked plum pulp (see above).
If you have a stick blender, pour 200ml plum syrup into a jug (or use a food processor or whisk). Add the silver leaf and whiz briefly in short bursts to small glittering pieces. Pour back into the rest of the plum syrup.
Soak the gelatine in cold water for 10 minutes, then discard the cold water and pour on 300ml boiling water. Stir until the gelatine has dissolved. Mix in the sloe gin, then stir into the plum juice. Chill in the fridge for about 2 hours, stirring regularly until the silver leaf remains suspended in the jelly.
Meanwhile, brush a 1.5-litre jelly mould with sunflower oil. Turn the mould over on a plate, leave for 10 minutes to remove the excess oil, then freeze until it is really cold to set the oil on the mould. Add the plum jelly and chill overnight in the fridge until set.
To serve, invert the jelly mould onto the centre of a serving plate and give a sharp shake. If the jelly is reluctant to come out, briefly dip in very hot water (for less than 10 seconds) and repeat the process.
Cook's tip: want to make a quick sparkly jelly? Use 1.1 litres of mixed cranberry and clear apple juice instead of the plum syrup.