For some reason I am no longer able to sell, drop or drink any of the alto wine in my inventory as apparently it is a quest item. Does anyone know which quest this refers to; or if this is a bug, a code to allow its removal?

A Night to Remember?

Have you checked your Misc quests? If you need to deliver one Alto Wine, if you have more than one in your inventory, I don't think it lets you drop any.

When you pick up the alto wine in the temple of diabella in Markarth during A Night To Remember it becomes a quest item and you have to finish the quest to drink/drop it.