I've never tried Cherry Heering and don't want to add seldom used bottles to my blooming bar (many are on the ground and some in my bedroom). I've searched for substitutions and haven't seen Luxardo cherry syrup from the jar of cherries as a sub. I understand that it's non alcoholic so maybe that's why, but in a cocktail like The Simon where it calls for only 1/4oz Heering I'd be interested to know what people who've tried Heering & Luxardo syrup think.
This discussion is quite old now, but I'd like to add that compared with Heering, Luxardo Sangue Morlacco will give you a cleaner, less syrupy cherry note. I prefer it & it's available here in Diffords shop too.