OK I finally got around to dealing to the quinces today. Chopped them and boiled them up with some water and will leave in the jelly bag overnight to drip juice.
They were as hard as rocks when raw, but after about 2hrs simmering in the pot they softened up nicely and there seems to be quite a bit of juice. Hopefully the jelly will turn out OK. Also smells nice. Have not tried quinces before so this will be a new experience for us.