Hi Neil,
A few years ago I was getting ready to replace my cockpit cushions when some friends just back from a circumnavigation suggested going without - just have a few small seat cushions that you can move around and use for your back as well as your butt. We decided to follow their advice and I'm glad I did. It's a simpler and cleaner cockpit. I was always turning the big cushions up on their side to let them dry from dew or seawater. When wet they could be a bit slick, and sometimes fell off the seats when heeled, or made it marginally harder to get to the lockers underneath ... to say nothing of the cost of new cushions.
I realize a lot of folks will prefer to have full sized cockpit cushions, but just mentioning that it isn't a necessity by any means.
All the best,
Max