Well, it would appear turning off nanites in the project settings cures this but of course isn’t a real answer as it leaves me with no Nanites and error messages regarding Nanites being not enabled. So, as an experiment I went to one of my large meshes and made sure Nanites was disabled for it, then switched Nanites back on in Project settings and rebooted. Went back to my large mesh which was happily sitting there and enabled nanites back on for it and the thing just vanished, one big hole where it should be! I guess some sort of shader rebuild is required but not sure how to do it. frustrating!