Thanks - I was searching the list of installed/available plugins, not the plugin repo.
I had a play with the convert formats with the KoboTouch Extended driver, but still couldn't see the option to convert to Kepub from inside Calibre.
However, I ran a test book through the kepubify tool (
https://pgaskin.net/kepubify/), merged the two books in my local library and then deleted the book from the Sage and re-uploaded in kepub. Absolutely night and day - page turns onto pages with code blocks have gone from 40seconds to instant. With this result, I'm going to hold off installing KOreader.
Thanks for your help!