Soak the gelatine in cold water for 10 minutes to allow it to rehydrate. Bring the milk and cream up to 80°C. Add the chopped celery and infuse for 30 minutes. Add the softened gelatine and blend with the celery cress. Season and pass through a chinoise into an ice bath. When cool and semi-set, place into airtight containers and leave for a minimum of 4 hours to set.