I have used mqtt_spy and can connect to and publish and subscribe to messages without error connecting to either the Pi2 or Pi3 Mosquitto broker. My mqtt_spy is running on my MacBook. So I am fairly sure it is not a Mosquitto issue. BTW the distro does not support websockets hence my build from source.
My ESP8266 has the same IP address when attempting to connect to either broker.
I also tried the PubSub library with the same results. The error code returned when connecting to the Pi3 broker was -2 !!
On thing I did try was to use the Ping.ping() library on my board and found that despite the ESP8266 being allocated an IP address I could not ping the new pi3 Mosquitto broker, hence the no connect! However when connecting to the old pi2 Mosquitto broker, it connected ok and clearly Ping worked. I could Ping the Pi3 broker from another computer without problems so I know it was online.
I will continue to investigate.
Thanks.