For anyone else looking in here.
Found an old RPI3+ in the box of spares. Used one of the official RPI PSUs and borrowed a 128gb Sandisk Ultra (with a warning not to thrash it) to test the install procedure.
Downloaded the RPI2 image, 7 zipped it and used win32 disc image to burn it. From start to booting GUI on the RPI3* took 15 minutes.
No errors reported on boot and definitely no under voltage detected messages.
No doubt if you’ve read through this whole topic you’ll have a general idea what to look out for in the event of a problem. Rule out the simple items first before digging down too deep and frustrating yourself.
The RPI3+ is back in the box and the 128gb is freshly formatted and safely back in the hands of its owner