Place the raspberries and strawberries in a heatproof bowl of simmering water, add the lemon juice and 50g of the sugar. Cover with cling film and leave for 30-40 mins. In a saucepan put the remaining sugar in the water and dissolve, bash the lemon grass and add this then bring to the boil. Take off the heat and leave to infuse for about an hour.
Strain the water into a jug. Pass the berry mixture through a muslin cloth or a sieve lined with bounty workes well too. Soften the gelatine leaves in water as per the instructions on the packet. Now add the berry juice to a clean pan and add the infused water. Bring to the point of boiling then remove from the heat. Add the gelatine and whisk through until dissolved.
Pass the mixture through through a sieve, add the champagne and leave to cool, when it is on then point of setting pour into serving glasses and chill until ready to serve