Mix the butter and biscuits, spread over the base of the prepared tin, press down firmly.
Place the redcurrants, sugar and water in a pan, bring to the boil, cover and simmer for approx. 10 mins. Puree the redcurrants, stir the gelatine into the hot mixture, pass through a sieve, allow to cool slightly.
Combine the quark, yoghurt and sugar, stir in the lemon zest and juice. Bring the water to the boil. Add the gelatine, dissolve, combine with 5 tbsp of the quark mixture and immediately stir into the remaining quark mixture. Fold the whipped cream into the mixture. Spread the redcurrants over the tart base, top with the quark mixture and redcurrant puree, create a marble effect using wooden skewers. Cover and chill for approx. 4 hrs. Cut the cake into squares, serve.