Additional crew members in Avro Lancaster

[Gregor30]

Posted on another forum but no answer so far, so I decided to post it here as well...

Maybe silly question, but I found it interesting to me :

The Avro Lancaster crew usually consisted of 7 members: a Pilot, a Flying Engineer, a Bombardier, a Navigator, a Wireless operator, and two Air Gunners.
However I found some notes in ORB of Lancaster Squadron, that sometimes The Station Commander used to fly for a bombing mission with a crew of any Sqadron he commanded.

My question is:
He usually was listed as II pilot (in a crew listing - after the name of a pilot and before the name of F/E). As there`s only one seat for a pilot in Lancaster, I wonder what was such an additional crew member was doing during a flight?

So if this was listed as II pilot, could it mean that he was considered a pilot only by politeness (as usually Station Commanders were pilots) or maybe he indeed had some duties to carry out during a bombing mission? In this case, what might have been his duties?

By the way, was there any place, a kind of unoccupied seat for such additional passenger in a Lancaster?