This jam is a delicious accompaniment to cheese, and it makes a wonderful glaze for pork, game or chicken. It's also great just on toast with butter! I suppose technically it's a jelly, because it doesn't have pieces of quince in it, but it's not clear like a jelly, so I call it jam ;)
Quinces contain a lot of natural pectin, so you don't need to add any. However, you will need to keep checking the set while you cook it - use the cold plate test to do this (instructions below in the recipe). Check the set after the initial 20 minutes of cooking, and then every 5 minutes after that. Do not give up - this jam has to cook for a while. It may take up to 40 minutes for the jam to reach the "set" point, depending on the level of heat you are using. Just keep cooking it down. The jam will turn a reddish-brown color as it cooks.