Solved!
I had set the permissions for the directory to server writeable and ticked the box to allow folder contents to be server writeable but they weren’t.
Individually set permissions on each cache file in folder and error message disappeared.
Thanks for the lightning response David. If I manually delete the contents of the cache folder and let them re-populate as the page loads, everything looks fine until at some ‘future’ point (the refresh cycle I guess) the error message pops up.
