'scuse this late reply. I sometimes don't get on here v. often.

Some unusual behaviour to report as well, after another attempt to use the scanner this week:

(i) I turned scanner/machine on.
System: recognised scanner.
Simple Scan: did not recognise scanner.

(ii) Scanner turned off. Left machine on for a couple of days.

(iii) Scanner turned back on.
Simple Scan: recognised the scanner.

(iv) Told Simple Scan to perform a scan.
Simple scan: performed a scan.

(v) Told simple scan to stop half-way through (had scanned as much as necessary for this test).
Simple Scan: stopped the scanner.
(But unhelpfully, the scanner stayed stopped at the point where it had reached in the scan - the scan arm did not return to its neutral position - another bug?).

(vi) I think I may have restarted Simple Scan at this point, in an attempt to get it to return the scanner to its neutral position.
Simple Scan: says  it doesn't recognise the scanner anymore.
scanimage: doesn't recognise the scanner either: "No scanners were identified."
Scanner: is still on, attached, and stuck where Simple Scan left it.

----------------------------------------------------
~/.cache/simple-scan/simple-scan.log

[+0.00s] DEBUG: simple-scan.vala:596: Starting Simple Scan 3.12.1, PID=8198
[+0.01s] DEBUG: Connecting to session manager
[+0.19s] DEBUG: ui.vala:1648: Loading state from /home/mark/.cache/simple-scan/state
[+0.19s] DEBUG: ui.vala:1629: Restoring window to 600x400 pixels
[+0.19s] DEBUG: autosave-manager.vala:64: Loading autosave information
[+0.19s] DEBUG: autosave-manager.vala:258: Waiting to autosave...
[+0.19s] WARNING: autosave-manager.vala:76: Could not load autosave infomation; not restoring any autosaves
[+0.30s] DEBUG: scanner.vala:1443: sane_init () -> SANE_STATUS_GOOD
[+0.30s] DEBUG: scanner.vala:1449: SANE version 1.0.23
[+0.30s] DEBUG: scanner.vala:1510: Requesting redetection of scan devices
[+0.30s] DEBUG: scanner.vala:802: Processing request
[+0.31s] DEBUG: autosave-manager.vala:280: Autosaving book information
[+0.41s] DEBUG: ui.vala:1739: Saving state to /home/mark/.cache/simple-scan/state
[+3.22s] DEBUG: scanner.vala:338: sane_get_devices () -> SANE_STATUS_GOOD
[+5.80s] DEBUG: autosave-manager.vala:194: Deleting autosave records
[+5.81s] DEBUG: scanner.vala:1583: Stopping scan thread
[+5.81s] DEBUG: scanner.vala:802: Processing request
[+5.82s] DEBUG: scanner.vala:1594: sane_exit ()

----------------------------------------------------
# simple-scan -- debug

(This was only running for the tail end of the procedure described above)

[+0.02s] DEBUG: simple-scan.vala:596: Starting Simple Scan 3.12.1, PID=21922
[+0.18s] DEBUG: ui.vala:1648: Loading state from /root/.cache/simple-scan/state
[+0.18s] DEBUG: ui.vala:1629: Restoring window to 600x400 pixels
[+0.19s] DEBUG: autosave-manager.vala:64: Loading autosave information
[+0.19s] DEBUG: autosave-manager.vala:258: Waiting to autosave...
[+0.19s] WARNING: autosave-manager.vala:76: Could not load autosave infomation; not restoring any autosaves
[+0.27s] DEBUG: scanner.vala:1443: sane_init () -> SANE_STATUS_GOOD
[+0.27s] DEBUG: scanner.vala:1449: SANE version 1.0.23
[+0.27s] DEBUG: scanner.vala:1510: Requesting redetection of scan devices
[+0.27s] DEBUG: scanner.vala:802: Processing request
[+0.30s] DEBUG: autosave-manager.vala:280: Autosaving book information
[+0.38s] DEBUG: ui.vala:1739: Saving state to /root/.cache/simple-scan/state
[+5.20s] DEBUG: scanner.vala:338: sane_get_devices () -> SANE_STATUS_GOOD