Soak the gelatine in 3tbsp cold water until softened. Then place the gelatine and water in a small pan and heat very gently just until it is dissolved - don't let it boil. Gently pour the Pimms and lemonade mix into the pan and stir until the gelatine has fully dissolved.
Divide the grapes between 4 glasses and pour the Pimms mix over. Leaving the skin still on, slice the satsuma into 8, then add a slice and a mint leaf to each glass. If necessary, scoop off any residue that rises to the surface when the bubbles subside.
Allow the glasses to set in the fridge for 2-3 hours before serving.