Preheat the oven to Gas Mark 6, 200°C, fan 180ºC. Put the digestive biscuits and sugar in a food processor and pulse until the mixture looks like fine gravel. Pour in the melted butter and pulse again. Pour the mixture into the base of a 25cm (10in) springform cake tin, press into the base and pack tightly flat with the base of a tumbler. Bake for 5 minutes, remove and set aside.
Reduce the heat to Gas Mark 3, 170°C, fan 150ºC. Put the cream cheese and the caster sugar in the mixer and beat for 5 minutes or until just combined and smooth. Add the eggs one at a time, beating well after each addition. Add the vanilla seeds, flour, cream and vanilla extract and beat until well combined. Pour into the tin and spread evenly. Bake on the middle rack for 1 hour, or until set but still slightly wobbly in the centre.
Turn the oven off and remove the cheesecake. Run a knife around the edge of the tin and return it to the oven, with the door ajar, to cool. Leave for 1 hour, then chill for at least 4 hours or overnight. Cheesecakes can crack when chilled, but don't worry if it does - you can cover them with the rhubarb.
Chop the rhubarb into 1in pieces. In a medium pan add the sugar, orange juice and rhubarb. Bring to the boil and simmer for 2 minutes. Soak the sheet of gelatine in cold water for a few minutes until it softens, squeeze the water out and add the gelatine to the rhubarb. Gently heat until the gelatine melts and the rhubarb is tender (about another 3 minutes).
Pour the mixture over the chilled cheesecake and chill for 1½ hours to set. Cut into slices and serve with some mint, if you like.