Six bottles of wine weighs 8kg. A Skoda Octavia Estate has a kerb weight of 1172kg -1415kg (including driver). So get your friends on the scales, add their weights to the "kerb weight" of the car (in the manual), subtract that from the "gross vehicle weight" of the car (also in the manual), and you will know how many kilos you can safely play with. Try to load as near to the centre of the car as you can, using the unoccupied rear footwell, for more even weight distribution.