To cut the story short, I'd previously moved mintara.prg sideways and never found it again, and so never configured the system to boot that way.
Adding a couple of lines to the Aranym config file:
Bootstrap = system/mintara.prg
BootstrapArgs = DEBUG_LEVEL=1 BOOT_DELAY=0 MEM_PROT=NO
and finding my existing mintara.prg
meant I was now booting with the right version.. The boot screen displays lots more and asks me about resolutions, except it then crashes with a bus error.. The last line in the logs is something about xaaes img files..
So I renamed the
rebooted and so got to the desktop.
The mouse pointer is still screwy but I'm guessing I'll do a fresh install soon.