Wash the limes under a hot tap with a clean, plastic scourer to remove any wax coating

Slice the limes in half and squeeze well reserving the juice

In a tall, stainless steel saucepan (aluminium can cause discolouration), simmer the limes in 1 litre of water for an hour

Using tongs or a slotted spoon, remove the limes from the saucepan and allow to cool for 5 minutes

Using a spoon, scrape out the lime pith putting it into a jam strainer or square of muslin. Knot or tie it tightly with string before putting it back into the saucepan of lime liquor

Slice the cooled, cooked peels very finely, add them back to the pan and top up with extra water until well covered

Reboil for another hour until the peel becomes very soft

Using the tongs or slotted spoon, scoop the peel out of the saucepan into a bowl or measuring jug

Squeeze the pith-filled bag with the tongs or back of the slotted spoon spoon as it boils

Remove the bag, add the sugar and bicarbonate of soda, stir until the sugar has dissolved and bring back up to a rolling boil

If you're using a jam thermometer allow the liquid to reach 105ºC/220ºF (this is the setting point for jam)

If you're not using a thermometer, put a saucer in the fridge

In the meantime, sterilise two 500ml jars & lids (or a number of smaller jars, enough to hold a kilo of marmalade)

Once the mixture has reached the setting stage, allow to cool for 15 minutes before decanting into the jars, screwing the lids on securely straight away