Okay, I get now that you are not meant to be able to attack ships in port. And generally I haven't been able to. But I went back to the save game where I thought it had occurred and indeed I was able to attack ships in port.

I had two navies, one already in the red zone of the port, although not right next to it (I had landed an army the turn before), and one some distance away. If I attacked with the landing fleet I just got a blockade; if I merged the fleets and attacked I got a blockade BUT if I attacked with the distant fleet I got a battle with my main (landing) fleet coming in as reinforcements. Whenever I tried to play this battle out I got a CTD about a minute or two in (repeatedly) but auto-resolving it worked okay.

Sorry but this is why I got confused. Anyone else had a similar experience?