There was a .htaccess file in my 'www' folder on the server. I removed it.
There was a couple of plugins activated. (Jetpack and WP Backup). I de-activated those.
I checked the ajax.php address and it showed a white screen and a '0' in the top left hand corner.
I re-ran the MVIS plugin .... takes around 90 seconds - 2 minutes to get to the screen telling me there are some issues.
I click on the Core Issues tab and the screen then goes white again (still has Dashboard on the left though). This takes a similar amount of time, maybe longer, as the initial check.
Once the Core Check screen opens I have a bunch of Green, Orange and Red Dots in the various File / Backend /System areas.
If I hover over one of the green dot 'i' icons it gives the No Risk message, if I click on it
I instantly get a black screen, white square, very very brief orange spinning timer and then in under a second the information appears.
If I hover over the arrows icon it gives me the issues identified message. If I click on it I I instantly get a black screen, white square, orange spinning timer and then no information.
I have previously left this spinning orange timer for over half an hour but get nothing back.