I can find no errors in the apache error log, except for a few times this one:
Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in /home/forum/public_html/forum/system/Db/Select.php on line 498, referer: https://forum.wscc.co.uk/forum/
Note, this error only occurred 3 times today, and none assigned to my own IP address. I however increased the memory_limit value in php.ini to 256M first, and now to 512M and with neither increased values has it mattered in regards to the uploading of my PNG test file.
I was hoping to see the GD error somewhere, but I've drawn a blank.