I wonder if the varying experiences of Rebecca/Eureka performance was directly related to the period of those experiences. I seem to remember that in the very early 60's they were fairly positive. We drew out range circles in different colours from the Eureka stations and then covered the topo' in clear Fablon. When airborne in the JP a range from one, and then another (and yet another?) would usually result in a fix (again, as far as I recall).
Could it be that the reliability of the ground stations (even transmitting at all?) could have deteriorated in later years? Just a thought...
As to the Zero Reader, it was even fitted to the Hastings, Beagle. Unlike my experience above, I do not have any positive memories of it. It shared with the Decca Log Mk1 (as fitted to the Hercules MkI) a requirement for you to hand feed it before it would grudgingly tell you anything of value.