Thanks for the suggestions everyone, and thank you Judith for checking the Index.
I just had a spare hour and so was going back through the threads here to refresh my memory and plan my next step. One of the early posts from JM caught my attention again when he mentioned that Thomas would have been free to marry upon receiving his CoF and that he could have moved to NZ or 'Australia Felix".
I came across a marriage reference:
Thomas WALKER & Martha GAMBALL
17 Jan 1843
Geelong
Presbyterian
V18431505 76
It is the date of marriage that jumped out at me - Thomas received his CoF 16 Jan 1843. Could be a complete coincidence of course but I hope to get time again tomorrow to follow up further, just thought I'd share this possible link.
I identified via
https://portphillippioneersgroup.org.au/pppg5g.htm that I could obtain a copy of the marriage certificate from the NSW Registry (if I decide it's worth pursuing, no "pdf" option).
I've only managed a cursory search on Martha Gamball. I found a Martha Gamble applying to marry in 1825 and a Martha Gamble arriving in Port Phillip in 1842 (she was Roman Catholic and 23yrs old, a lot younger than my Thomas) but no Martha GAMBALL (I know spelling was fairly flexible though).
Thanks again for all your help and suggestions, it's keeping me motivated to chip away at my search whenever I can!
Caroline