Have ready 8 lightly oiled individual moulds.
Put 3 tablespoons very hot water into a small bowl and sprinkle over the gelatine. Stand the bowl over a saucepan of hot water and heat gently until dissolved.
Meanwhile put the cream and caster sugar in a saucepan and bring the cream almost to boiling point, stirring to ensure the sugar has dissolved. Remove from the heat and stir in the elderflower. Then whisk in the Philadelphia until melted. Then stir in the dissolved gelatine. Divide the mixture between the moulds and chill for around 4 hours or until set.
Meanwhile put the plums in a saucepan with 50ml water, brown sugar and cinnamon. Heat gently stirring occasionally until the sugar has dissolved, then simmer for 7-9 minutes or until the plums are cooked. Chill well before serving with the unmoulded panna cotta.