Just to feed back for those interested, I followed the suggestion posted by little britain and the whole day went like clockwork. We cancelled the 8 return RCI transfers while on-board and got a full refund so booked with Bags&Go to take all our luggage (11 bags). Collected our bags in the terminal at 8:30, wheeled them a few feet across to the Bags&Go desk and dropped them off, then hopped onto the Cruise Bus into town and walked up La Rambla to Placa De Cataluna. Three of us got a taxi to the Camp Nou football stadium and took their basic tour, then went back to Placa De Cataluna to meet the rest of the family. The grandchildren loved feeding the pigeons so that made their day. Finally we took the Aerobus back to the airport (pre-booked that too) at 5pm where Bags&Go were already waiting with our bags all loaded onto trolleys ready for us to wheel away.
Superb service all round and turned a potentially boring day stuck in the airport into a bonus day in Barcelona. Great suggestion from little britain !!