After about twenty hours in the the oven with just the light on the peel was crisp and dry and utterly delicious.
I put some in jars and the rest I chopped up fine for the mincemeat. The jar on the left has the larger pieces of grapefruit peel.
Without the sulphites the colour will probably darken, particularly with exposure to light. I'll keep an eye on it and re-post any significant changes. It should still taste good even if it does go brown.