Hi Steve,
Many thanks for your timely response 
The link you posted only lists software updates, not firmware updates.
For example:
- 'jfirmware-srxsme-12.3X48-D65.1.tgz' is the firmware file needed to update the VDSL PIC
- 'junos-srxsme-12.3X48-D65.1-domestic' is the software update for the JUNOS OS.
I have updated my JUNOS OS and now need to update the VDSL PIC to 2.16 so that I can take advantage of G.993.5 Vectoring for VDSL as per the KB article: https://kb.juniper.net/InfoCenter/index?page=content&id=KB31818&actp=RSS
I found a link to a jfirmware file, but it is for version 18.1R1 not 12.3X48: https://www.juniper.net/support/downloads/?p=junos-srx
I am hoping someone in the community can point me in the right direction.
Thanks
Alex