I loved some of these lists, sounded like the BBQ was going to last for a week, not just for a day.
I would say pack light and enjoy yourself, unless you plan for everyone to go swimming in a full set of clothes then swimming costumes and a towel will be all the extra clothes you need, you can even sit on the towel to eat.
Plastic plates, cutlery, bread roles and tools for the BBQ are a good idea tho.