To sterilize the jars

Wash the jars and lids with warm soapy water and rinse (see Cook’s Notes).

Turn on the oven at 140°C/275°F/Gas Mark 1. Fan Oven 120 °C.

Place the jars and lids upside down onto the oven racks and leave until required.

To make the conserve

Wipe clean the rhubarb, chop into small pieces and place into a large non-metallic container.

Tie the elderflower heads securely into the muslin square to make a small bag.

Zest and juice the lemons into a small bowl, discarding the pips.

Add the elderflower bag or concentrate, lemon zest, juice, vanilla extract and the sugar with pectin to the rhubarb and stir. Cover with cling film and put in a cool place overnight.

Transfer the soaked fruit and contents to the 3-ply Stainless Steel Preserving Pan and bring to the boil over a low to medium heat, stirring until the sugar has dissolved.

Maintain a slow, rolling boil for 40-50 minutes. Stir occasionally at the start but more frequently towards the end of cooking (see Cook’s Notes).

Test for a set (see Cook’s Notes). If the conserve is not ready return to the heat and boil for 5 minutes more and test again.

Cook's Notes

Sterilizing the jars. If using jars of various shapes and sizes, add a few extra to be prepared. To sterilize a jar quickly wash and leave very wet, microwave on full power for 1 minute. Always use a long-handled spoon or spatula when stirring. Take care towards the end of the cooking process when the conserve becomes thick and may bubble up.

Methods to test the set

Temperature Test - To test the set, stir the conserve well to distribute the heat and place the base of the sugar thermometer into the conserve. A good set will be achieved at 105°C/220°F. Sugar thermometers are relatively inexpensive and can be found in most good department stores and cookshops.

Plate Test - Place a saucer or small plate into the refrigerator before starting to make the conserve. Once it is ready to test drop a small amount of conserve on to the cooled dish and wait for a minute or so. Run a finger across the top of the sample; if the surface wrinkles it should be done. Remove the preserving pan from the heat whilst awaiting the result to prevent over boiling.

Flake Test - Stir the conserve with a large spoon, lift the spoon out of the conserve and turn a couple of times to cool a little. The conserve should form a droplet from the spoon and partly set on the spoon; some will fall forming what are known as “flakes”.

Most cooks prefer to use more than one test as acidity; sugar and pectin levels can all affect the set.