Wash the rhubarb stems and cut off the leaves and the stem bases. Cut them into chunks about 2 cm long.
Put the chunks into a bowl in layers, adding a little sugar over each layer and putting in the elder flowers head-down before doing the last layer.
Pour the rest of the sugar over the top and leave overnight.
Take the flowers out and steep them in water to make an instant cordial.
Boil up the jam in the usual way. Between the rhubarb and the lemon, this jam will set well so there is no need to overdo the cooking.