Thanks for your help so far - I hope it's not too awkwards to remedy. Good luck!
I did spot that the image had been rebuilt today (and the checksums were different to yesterday's image) - but with the same outcome, albeit only the one segmentation fault in syslog. Output here if it helps:
Mar 26 19:45:54 cdrom-detect: CD-ROM mount succeeded: device=/dev/sr0 fstype=iso9660
Mar 26 19:45:54 cdrom-detect: Detected CD 'Debian GNU/Linux 9.0 "Sid" - Unofficial sparc64 NETINST 20180327-13:13'
Mar 26 19:45:59 cdrom-detect: Detected CD with 'unstable' (sid) distribution
Mar 26 19:45:59 anna-install: Queueing udeb eject-udeb for later installation
Mar 26 19:45:59 anna-install: Queueing udeb apt-mirror-setup for later installation
Mar 26 19:45:59 anna-install: Queueing udeb apt-cdrom-setup for later installation
Mar 26 19:45:59 anna-install: Queueing udeb sid-support for later installation
Mar 26 19:45:59 main-menu[248]: INFO: Falling back to the package description for brltty-udeb
Mar 26 19:45:59 main-menu[248]: INFO: Falling back to the package description for brltty-udeb
Mar 26 19:45:59 main-menu[248]: INFO: Menu item 'load-cdrom' selected
Mar 26 19:45:59 cdrom-retriever: warning: File /cdrom/dists/sid/main/debian-installer/binary-sparc64/Packages does not exist.
Mar 26 19:45:59 kernel: [ 119.665106] anna[1116]: segfault at 0 ip ffff800100333c000+160000]f800100234f74) sp 000007feff8aa981 error 1 in libc.so.6[ffff8001003
Mar 26 19:45:59 main-menu[248]: (process:1114): Segmentation fault
Mar 26 19:45:59 main-menu[248]: WARNING **: Configuring 'load-cdrom' failed with error code 139