I soon realised (by repeated attempts) that I was never going to be able to boot the USB either via the Mac boot manager or rEFInd. I downloaded the Ubuntu Server 15.04 server ISO and then created a bootable USB drive on my Windows 8.1 laptop with Rufus for Windows (although I suspect the Universal USB Installer on Windows and the Startup Disk Creator on Ubuntu would have worked as well). On my Mac, I started with a reasonably fresh but basic install of OS X 10.5 with the rEFInd boot manager installed. The guide which I followed and which, with a few tweaks, got me up and running, is by Jason Heeris. The technical background is available at the rEFInd Boot Manager site. Requiring the use of the Mini as a server once more, I turned to the challenge of installing 15.04 Server, single-boot (no OS X), in pure EFI mode, using only the Mac EFI boot loader. However that install never worked correctly to my satisfaction.
At some point I did manage to install a version of Ubuntu Server (I think it was 12.04) alongside Mac OS X, booting in BIOS emulation mode, using rEFInd. I've had some problems getting Linux to install or even boot on this machine. I am posting this for the benefit of the community, in case it helps someone else along the way.
