UPDATE:
Rather than taking out the unit and sending to auto electrician, I did something just as “cheap”. I got a replacement head unit online (eBay car dismantler) as a donor unit for £150. Had guarantee that was in working order so took a chance.
Took off front panel and fitted to connectors on my head unit and all works correctly.
Just swapped out circuit board (although I could have simply swapped front panels but they are different colour, mine is black whereas donor has a brown hint to it) and fitted. All working now so result!
I will eBay (it’s a 2006 ML unit) the donor head unit (non working Audio button of course!) to see if I recuperate some cost.
note I did not take photos of the process but if anyone wants more detail as to how I did this, I am happy to advise.