@Kim Shook, the crackling on this pork was really good and I think that came down to the method. It was certainly a lot better than my usual attempts!
as others have suggested, separating the skin from the meat is very helpful. In this case the sheet of skin was cooked SV at 70°C for 24 hours. After this time it was seriously soft with all of the collagen broken right down. Before starting there was very little fat on the underside which I think was helpful.
I reapplied the skin to the cooked meat before crisping but I’m sure it could have been cooked separately too. Maybe dried the sheet in the fridge overnight before using the broiler or a very hot convection oven.
edit:
this is the skin before and after SV:
mane reuinted with the rolled belly before tying and blasting at 230°C:
