You will need to purchase the Line-In adapter and set the adapter up for Autoplay before you take it off the system. 

https://www.sonos.com/en-us/shop/sonos-line-in-adapter

https://support.sonos.com/en-us/article/play-line-in-on-your-era-100

https://support.sonos.com/en-us/article/adjust-line-in-settings

Beaten to the answer!  

Thank you for taking the time to respond. Can you confirm though that once i have it all set up that i can then play without bluetooth just via the aux in lead?

Did you read what it says in the links? Does that not answer your question?

Hint:

“You can also enable Autoplay on Era 100 to automatically play your Line-In source when Sonos detects a signal. See our article on adjusting Line-In settings for more information.”

I did yes thank you however, the links do not confirm if you still need to be connected to wifi or 4g to use the app or if it will work on downloaded music.

As long as Autoplay is set to On and the Autoplay room is the Era 100, you can play the Line-In without being connected to the internet or WiFi.

Note that the line in will be looking for a line level input, the output of your headphone jack will be a variable input. This will require some ‘fiddling’ to get to work, you certainly don’t want to try it the first time at the event. Test it at home first, and I’d recommend setting the phone’s volume to 50% to start with, and make adjustments from there.

This is not what the system is designed for, you’re likely to run into various speed bumps while setting it up. None that would be showstoppers, though. 

That’s great thank you so much.