Check the JAR licence for an expiry date. The document itself needed to be renewed every five years so it is most likely past that date. Also, as you didn't convert it to an EASA licence before 8 April 2018, you are now unable to exercise the privileges until you convert to an EASA licence. See here:
https://www.caa.co.uk/Commercial-ind...r-JAR-licence/
I can't comment on the pros and cons of using your Australian licence as a basis for conversion, but would suggest contacting a flight school about this. As you're current on your Australian licence, they may be able to sign off on a minimal amount of training to get you up to speed for a SEP prof check.