Complex sentence: The left-hand position was the main one, with an instrument panel and a fixed seat; the right-hand, co-pilot's seat could be folded away to allow access to the nose gun-position via a crawl-way.
Split into simpler sentences: The left-hand position was the main one, with an instrument panel and a fixed seat. This could be folded away to allow access to the nose gun-position, via a crawl-way.