Hi AusEng,
Welcome to the forum !
Here is a knowledge base from Juniper explaining the extra steps you have to take to create an IPSec VPN tunnel between two peers when one of them has a dynamic IP :
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28108&smlogin=true&actp=search
You might also find this tutorial ( and site ) helpful for Juniper SRX configurations :
https://www.fir3net.com/Firewalls/Juniper/srx-dyn.html
The articles assume you already know how to configure a site to site IPSec VPN tunnel between two SRXs with static IPs and starts off from there, so if you've never used Juniper SRX you will have to read up on it 
.