Put the syrup, sugar and vanilla / almond extract in a heavy bottomed pan and cook for 10 minutes on a medium-low heat
Add the cherries (I like to leave them whole but you can halve them if you like) and cook for a further 20 minutes until broken down and thickened. The liquid needs to reduce by a 3rd
Add the lemon juice, stir in and take the pan off the heat
Take a small spoonful of the jam and put it on a cold saucer. Let it cool a minute then push it with your finger. If the jam wrinkles as it slides across the plate, it's done. If not, put the pan back on the heat and check again in 5 minutes
Pour into a hot, sterilized jar and let cool before keeping in a dark place
Using fresh Morello or sour cherries, use twice the weight of the pitted cherries in sugar, mix together in a pan and let sit overnight. In the morning heat on medium-low until thickened as before, adding the lemon juice at the end
