In a large pot (enough to hold 2.5litres of liquid in total) place a ham/bacon joint, cover with cold water, bring to the boil, and simmer for approximately 1.5 hours. Switch off the heat and cover the pot leaving it overnight for the flavours to marinate.
In the morning
Remove the bacon joint from the cold water and set aside, skim off any fat from the liquid in the pot. Chop off little pieces of the bacon meat (do not add any bacon fat) from the joint and add back to the liquid. Bring the liquid back to the boil and leave simmering gently. Meanwhile prepare the vegetables.
Bring to the boil and turn down to simmer the 1.5 litres ham stock (with bacon meat pieces if used)
Add the red split lentils, chopped leeks, chopped potatoes and diced carrots.
Bring all the ingredients back to the boil and simmer gently for 30 minutes, stirring occasionally. Add boiled water from your kettle if required to top up.
Once all the vegetables are extremely soft, turn off the heat and allow the soup to cool for about 30 minutes.
Use a potato masher to mash all the veg. Do not puree the ingredients; just mash them down to a consistency you like. Add boiled water from your kettle if you need to add more liquid.