Add enough water to just coat the base of a large pan (there will be loads of liquid that comes out of the red currants).
Add the redcurrants (stalks and everything) to the pan and place over high heat. Bring to the boil, then boil for about 8 minutes or until soft enough to pass through a sieve. Remove from heat.
Pass all the redcurrants through a sieve. Discard the skins, stalks and anything else left in the sieve. Add the redcurrants liquid back to the pan and add the sugar, lemon and dried spearmint.
Bring the liquid to the boil, then reduce to a strong simmer. Let reduce for about 30 minutes or until the liquid starts to set.
Place a plate in the fridge to chill. Keep testing whether the jam is ready by pouring a spoonful on the plate and put it back in the fridge for a minute or two to cool. If the jam is runny, keep boiling the jam. If it sets on the plate and goes wrinkly as you push your finger through it, then the jam is set.
Pour into sterilised jam jars and seal. Leave to cool, then store in a cool, dark place till opened.