Pour the coconut cream and double cream into a heavy-based saucepan with the sugar, broad strips of lime zest and crushed coconut, then set over a low heat, stirring a little to help the sugar dissolve. Turn the heat off just before it boils and leave to infuse for 1 hr (longer is fine). Strain and throw away the solids.
Soak the gelatine in cold water for 10 mins to soften. Reheat the coconut mixture until hand-warm (if it’s too hot, the panna cotta will not set properly). Stir in the lime juice.
Lift the gelatine from the water and squeeze out the excess liquid. Stir into the coconut mixture. Divide between four metal moulds, each with a capacity of 125ml, or up to six teacups or small bowls if you’re not turning out the panna cotta. Leave to cool, then put in a small roasting tin (just to transport them). Cover and chill in the fridge for about 4-6 hrs.
Cut off the two ‘cheeks’ of the mango. (You could use the rest of the flesh for a smoothie). Place them, cut-side down, on a chopping board, then cut on an angle into thin slices. Transfer to a bowl and pour the lime juice over them.
If you want to turn out the panna cotta, dip the moulds in to a bowl of boiling water for 5 seconds and loosen the edges with a knife. For each one, put a plate over the mould, turn the mould over while holding the plate in place, and shake. The panna cotta should slip out. Garnish with fine strips of lime zest, if you like. Serve alongside the lime-marinated mango.