Line a sieve with cheese cloth, put over a large measuring jug or a saucepan, and set aside OR get a jelly bag ready.
Core and seed the red peppers, cut into large chunks, put into food processor along with 100 ml (½ cup / 4 oz ) of the cranberry juice and using the pulse button, whiz into fine chunks, but not into a paste or purée.
Empty the food processor contents into a medium-sized saucepan. Add the remaining juice (that's 775 ml / 3 cups / 24 oz, if you are tracking the math ) and the teaspoon of red pepper flakes. Bring to a rolling boil.
Remove from burner immediately.
Pour into jelly-bag or cheesecloth-lined sieve from step 1. Let stand for 1 to 2 hours.
Discard contents of the sieve.
From the jug or saucepan, measure out 1 litre (4 cups) of liquid.
Put liquid into another saucepan. Whisk in the cardamom (if using), the pectin, and the vinegar.
Return the saucepan to the burner. Add the margarine if using.
If using STEVIA: Bring to a full rolling boil over the heat and when it reaches that point, start timing 1 minute. Remove from heat. Add stevia to taste.
If using SPLENDA or SUGAR: Bring to a full rolling boil over the heat and when it reaches that point, start timing 1 minute. Add in Splenda, bring back to a boil that you can't stir down, and boil for 1 minute after that point. Remove from heat.
Add a few drops of red food colouring if desired for a deeper glowing colour (if you use red cranberry juice, you probably don't need it.)
Ladle into either 125 ml (½ cup) or quarter-litre (½ US pint) jars.
Leave 1 cm (¼ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jar for 10 minutes; increase time as needed for your altitude.