Heat the currants gently (no water) until a mush, but don't boil. Strain through muslin, but a fine sieve will do. 1 lb sugar to 1 pint liquid, make sure the sugar is dissolved, then boil until it hits jelly on the thermometer. If no thermom, then boil 5 mins, remove from heat and put a teaspoon of jelly onto a cold plate (i.e. been in fridge). Pop it back in the fridge for a minute or so and then push a finger through it. If it wrinkles, it's set. Jam jars through the dishwasher and then dried in oven to sterilise.
To be honest, set or not set doesn't matter much if it is for adding to gravy, it all tastes the same. Avoid over boiling as the sugar can caramelise and tastes burnt. You can increase yield by pushing the mush through the sieve, but it can make the jelly cloudy, but this won't matter unless you are entering the local WI competition!