A6 Allroad C7 MMI failure (Jap import)

Hi everyone, new owner (since earlier this year) of a 2012 A6 Allroad 3.0TFSI, imported from Japan.

Mechanically it's running like a dream - only 60k miles, full inspection and service carried out at Renn in Iver. I've also had a stage 2 tune from MRC, with IE air intake, APR coils, MRC supercharger pulley, ECU & TCU remap... all of which gives it a slightly ridiculous 494bhp.

Unfortunately, it seems to have some electronic gremlins... First off the passenger wing mirror wouldn't fold out. Eventually got it out and have now just set the mirrors not to fold, problem solved! The next problem is a bit more inconvenient though, with the MMI system not starting up at all. It started as an intermittent thing, gradually becoming more regular and now has not worked at all for at least a month. The screen doesn't pop up, the button on the centre console don't light up... nothing. I've used fibre optic loops to isolate the various components int he boot, but no dice. Also tried the hard reset, but that does nothing either. Therefore, I'm assuming the head unit is the point of failure, unfortunately.

I found a link with steps for reloading the firmware, but it seems a bit out of my comfort zone, so I'm resigned to the fact that replacement is needed. I'm guessing I need an Audi dealer to make it work once a new (second hand) one is installed. My real issue is knowing which units will work. Mine is 4G0 035 678 C which, I believe, is a Japanese specific version. Would I be ok using a (more easily available) UK version? Does anyone know which part codes would be compatible?

Alternatively, is it worth going to an auto elec specialist to see if they can do anything with it?

As a side note, I also have some rattling from underneath when I go over small bumps - I thought it was the small plastic covers behind each wheel, as they were very loose, but I've replaced those and the rattling remains. It doesn't seem like it's a direct suspension component as the rattles aren't entirely linked with bumps and/or suspension travel, but I could be wrong. Are there any "usual suspects" I should look at first? It would be easier to investigate this if I could get the MMi working and therefore change from dynamic to allroad mode and get better access underneath!

Thanks in advance for any assistance.