Mail Call...
Received another couple of pieces to the puzzle. This is a Ryobi One DIY adaptor and DC-DC converter to reduce the 18volts down to 12volts. This allows you to use a Ryobi battery for the power (electric start / lights / horn etc).
I noticed these parts listed in Steve Vinsons Amazon list that was linked to from his YouTube videos. It seemed like an elegant solution to a problem I was yet to encounter so I searched for the same parts listed locally on eBay and placed an order.
If anyone is interested here's the parts I purchased.
https://www.ebay.com.au/itm/314275469574
https://www.ebay.com.au/itm/383495785534
The battery holder needs some kind of bracket to stop it from rattling around but that should not be too much of an issue.