Goal Defeat Three Scars in Black Storm at Cool Ranch (Big Sky).
When I approached the storm and the dialogue box popped up, it said "Group Members Only". When I entered alone and with one other ship, the target ship was labelled "Unsinkable" just underneath its name after you click on it.

I wonder if you need more than two ships in a group just to be able to sink it.
Can anyone shed any light on this?
I have no other quest to do so I'm really stuck in the game until I can get past this hurdle.
Thanks for any help you can give me.

Yep, "unsinkable" means that enemy will only fight you on-deck using a battle board. You can't attack or sink their ship at all. Just go up to them, board them and fight it out.