Wash and pat dry the fruit. If the strawberries are large, cut them in half. Remove the stalks from the redcurrants by gently running the tines of a fork down the stalks.
Put the fruit in a medium sized pan along with the sugar, lemon juice, and 540 ml (18 fl oz) (2¼ cups) cold water. Heat gently, stirring until the sugar has dissolved and the liquid is beginning to bubble. Now put a lid on the pan and let the fruit continue to cook gently until it has become very soft, about 5 minutes.
If using leaf gelatine prepare it now. Place the leaves in a bowl of cold water, snapping them in half to fit if necessary. Leave for 5 minutes to become very soft.
Now strain the fruit mixture through a large, fine sieve into a bowl. Press the fruit through the sieve, using a wooden spoon, to extract as much juice from the fruit as possible.
If using powdered gelatine, stir the contents of 1½ sachets in ⅓ cup of cold water then add ⅓ cup of the hot fruit juice and stir until the gelatine has completely dissolved, about 2 minutes.
Either lift the softened gelatine leaves from the water, squeezing out the water, and add to the fruit juice. Or stir in the dissolved powdered gelatine. Stir until the gelatine has completely dissolved into the fruit juice, about 2 minutes. Keep stirring until you are sure the gelatine has dissolved, otherwise, when the fruit juice sets, there will be little grains of solid gelatine in it.
Pour the jelly into the jelly mould or individual glass dishes and leave to cool. Cover and refrigerate overnight to set.
To turn out the jelly mould, dip it halfway in a large bowl of hand hot water for about 5 seconds. Then place a flat serving plate on top and turn out the jelly. Shake for a couple of seconds until you hear the jelly loosening, then lift the mould away.
Can be served with single/table cream or dollops of whipped whipping cream/whipping or heavy cream and extra fresh raspberries.