I have my A7C set to back button focus (using AF-ON button) and half press disabled. I am using "Tracking Expand Flexible Spot" focus area which I like for street. The problem I am having is after, the square focus block has tracked the subject, which can sometimes be to the edge of the frame and if I release the back button it stays there and from then on I can't move the square back to the centre. Any attempt to use left, right, up, down on the joystick just takes me into the function for the joystick. Occassionally it plays ball and moves for me, but I can't fathom out why, but 80% of the time it stays where I don't want it to be. I am assuming I am doing something wrong, but don't know what. Is there any way (in order of preference) to: Make the joystick control the focus square when I want it to? Make the focus square default to the centre of the screen? Disable the functions on the joystick so it only controls the focus square? Any help much appreciated      

Are you sure you haven't assigned some strange setting to the front or rear dial? If it were me, I'd check all my settings. If all else fails, do a factory reset. 

It really doesn't tell you anything, your assumption is based on a very scant bit of evidence. I use an A7 IV as my backup and have shot it in much, much colder temperatures than that, well below freezing. I've shot it in rain, snow and brutally hot temperatures, next to pavement that scanned 140°F in direct summer sun. There would be no point in manufacturing a camera that couldn't operate in such normal temperatures. Something else was going on.