All-Thanks for the replies. The video link supplied by factoryrat was exactly what I needed to see. It's the plug (part #10090) that seems to be stuck. In the video, the driving spring plunger plug has to be hammered into place. As Dfariswheel suggests, the plug has indeed been battered into place, so some force will be needed to remove it.
As mentioned previously, this is my first post. I am very grateful for the replies, and very impressed with the knowledge to be found on this forum. Thanks again!