Place the raspberry jelly cubes in a measuring jug. Add 225ml just boiled water and stir until the jelly has fully melted. Pour half the liquid into another measuring jug. Set that jug aside and top the remaining jug up to 275ml with cold water, then divide the more diluted mix between the two jelly moulds and chill for 4 hours until fully set.
When the first layer has set, top the reserved jelly up to 300ml with double cream – if it has started to set, heat in the microwave for 20-30 seconds. Divide the creamy jelly mix between the two jelly moulds on top of the first layer and chill for a further 2 hours.
Dip the base of the jelly moulds in a bowl of hot water for 10-15 seconds to loosen, then gently tip out and serve.