OK, Sigil crashed again and this time it deposited a dump file as directed. However the .dmp is 4MB! Opened with Metapad (warned it was a binary file but opened as text) lots of garbage (of course) and space with readable text sparse, and not being adept, couldn't gather much from it. Installed
WhatIsHang... a program that runs in the background and detects a program when it hangs, supposedly giving info about the cause. Will try that now....