More than likely a network storm caused by STP.  If the Ethernet port is connected to a switch which is not configured for STP, the dual wired/wireless connection can cause a loop which causes a blizzard of network packets.  You can try to configure the switch/port correctly, or an easier solution is to turn off the WiFi on that Play:1 to prevent the loop. 

All the Sonos components go through the same switch so its configured correctly (I dont believe you have to do it port by port)

I cant turn off the wifi (Ive tried) on the new Play 1 as it needs to wired to do that which I cant do as it brings down the network…..

Temporarily power off the other units. Wire the Play:1. Disable its radio. Power the other units back on again.

Alternatively, disable the radios on each of the units other than that Play:1. Wire the Play:1. Disable its radio.

Ill try checking the switch (though I dont think thats the problem) and try powering down all the others and power up thi one first.

All the others already have the wifi disabled though

Ill report back

If all the others have WiFi disabled, that would explain why you weren’t seeing any network storms before, regardless of the configuration on the switch.

If that’s really the case a loop shouldn’t be able to form. 

If the problem with the Play:1 continues then try plugging it into a different switch port, one temporarily vacated by another of the Sonos devices.