Cut the top off the pineapple, and use a spoon and small serrated knife to remove the flesh, leaving a one-centimetre shell all of the way around. Roughly chop the flesh and put it into a blender along with the caster sugar, rum, coconut cream and lime juice.
Pour the liquid into the pineapple shell - you will have some left over, put this into a tupperware and put this and the pineapple into the freezer for two hours.
After two hours, use a fork to scrape and mix up the semi-frozen liquid inside the pineapple. Repeat every hour until the juice has turned into a granita; depending on the size of your pineapple this could take 4-5 hours.
After the first three churnings the pineapple should be fine to rest in the freezer overnight - so you can always make it ahead and finish on the day you need it.
Serve the frozen pina colada straight out of the pineapple or in a martini glass, decorated with a sprig of mint, a cocktail cherry and a cocktail umbrella if you like.
