Ford Focus 2011 Sony Radio Bluetooth issue

[JNolan]

Good evening all. Bit of a long one to explain the cause and symptoms, but here we go. I have an issue with the Aux and Phone functions of my Sony DAB radio unit. 

Issue started when I had an electrical issue on the motorway. Driving along and I got a few warnings, beginning with "Hill start assist not available", and also included something about a voltage issue (maybe more relevant). This put the car into limp mode, but pulling over, stopping, waiting a bit, and then starting put everything right with the exception of the radio etc., which was all dead (no screen, nothing). The issue has occurred neither before nor since, and there have been no other symptoms. 

Fast forward a radio-less year, and I eventually bought a second hand unit with LCD and buttons off eBay. I replaced the radio unit. No joy. Replaced the buttons, no joy. Replaced the LCD, and everything sprang back into life. I even reinstalled the original radio unit and buttons and it turns out that the LCD was the only issue. 

Hooray, I have a radio. My phone connected via bluetooth, and I could hear calls through the Speakers, so I can be 100% sure that there is a bluetooth connection. 

However, if I press the Aux button, it mutes for a fraction of a second, the menu flickers, but returns immediately to radio. The phone button does nothing. (see video here: https://1drv.ms/v/s!An6VJFGGmT1Cp_Z3HAEse4d9KxcHaw)

If I try and pair a phone, I get the audible "enter the pin displayed on the screen into your mobile phone" message, but still the display remains on radio and so I cannot see the pin. 

I simply cannot fathom the issue and I'm hoping providing this detail can trigger inspiration from someone with more talent than me for these things. 

Can I reset the system? I've tried to research how to reset the SYNC system but cannot find anything that seems to be relevant to this model. 

I've disconnected the battery for an hour to no avail. I feel like there should be a simple fix but it escapes me for now.