@re0:
Many thanks for the response and suggestions.
Yes, the RPi Zero W is using a Wi-Fi connection to the router, the Pi being velcro'ed to the casing of the Billion and powered from it's USB socket, I do have a micro USB to ethernet adaptor for the Pi which I can use to directly connect to a router port but to keep things simple I only use this when setting up the system, configurating the Wi-Fi port etc. in the first instance.
Using normal, same premises, connection to the Pi, ie., from local desktop PC via VNC which is connected to the router by a pair of mains home plugs but still using Rpi Wi-Fi gives instant connection every time.
Looking at the situation simplistically, the only thing that has changed is the FW update on the Billion router hence the "likely" conclusion that this has caused the change in connection behaviour.
As the random re-booting of the router certainly appears to be cured with the FW update, over a week now uninterrupted DSL connection, I definately won't be rolling back the FW revision to prove / disprove the point, will likely just live with the minor inconvenience, connection is usually established after a few tries at most and remains steady for the duration of the session once established.
Will check the syslog on the Rpi, had not thought of that to see if that offers any clues.
Edit: Typo correction