Ist of all a big Thumbs up to Anker. After raising the fault and sending a video of the unit I was given a full refund and told no need to return the dead unit. Appart from the disappointment getting a swift refund has allowed me to order a replacement, a Soundcore Boost. Before I put the dead Pro in the general waste I decided to remove the Lithium battery to dispose of in a recycle bin. As soon as I popped the top off I could smell electrical burning. After full disassembly, I got to the battery. Nothing inside appeared burnt or damaged, in fact it appeared perfect. Looking at the battery once removed it is labeled 3.7v 14.8wh.
I put a multimeter across the terminals and was very surprised to see a reading of 8.8v. Maybe this was the issue. Presumably, the battery is made up of cells and maybe they are wired wrongly in series instead of parallel. Anyway, the battery has a good charge so I will keep it as a souvenir to power a camping light. Not sure how I can charge it though.