Thanks all, for your replies.
I have to admit taking the pram cover out cruising wasn't originally the plan, because I don't like it getting in the way when folded down, either on the rail or on the roof. So I was thinking I'd just store it somewhere else when we're out - but it's great having sitting out space when it's chilly or drizzly so I started wondering whether it would be possible to take, but be able to store away neatly.
It's already proving its worth when on the mooring, keeps the boat much warmer and the extra space is great.
Pete & Helen, I like your idea of a joining section, will look into that.
Jan