With the case on and no heatsinks the Apollo stopped hashing at high temperatures (40 degrees C ambient).
A restart fixed the issue, but this happened to me a few times in ECO mode with the fan on auto.
I think the fan could not cool the Apollo enough at high temperatures as it is only rated to run up to 40 degrees C ambient.

So I added all the heatsinks and improved the cooling in my loft.
Both of these really helped and the heatsinks stopped the fan intermittently surging to high rpm which was annoying.
Now my loft ambient peaks at 33 degrees C ambient and the Apollo is very quiet.

As the Apollo is not so cheap I wanted to protect my investment for a long life so added the heatsinks and removed the case.
I would recommend others to do this if they are running the Apollo above 35 degrees C ambient.