Update.
On a suggestion I contacted the NRMA's Motoring Advice service to see if they had any ideas I'd not yet explored. The lady was very helpful, although she didn't have great news. Basically, Australian Consumer Law is weaksauce; it only covers direct consumers. Because of the chain of ownership the car has been through, I am not Ford's problem; their obligation stopped with the first owner. Not to say it wasn't worth trying with my letter, but there's nothing in law to compel them to fix the fault for me.
She did have one ray of hope for me though: since the previous owner and I relied on the service history and particularly the condition report on the last service invoice (which, remember, said mentioned a hesitation but said that all was well) to agree to the sale, I could try arguing that the condition of the car was not accurately represented by Ford (who should have known there was a latent fault behind it). Still a long shot but at least it's something else I can try.
However! I've since got in touch with the previous owner, and I might have better ammo. He kindly obtained and sent through to me this complete service summary from the dealership (apparently they don't keep detailed records after 12 months). It's in reverse date order, with most recent at the top. Note the items at lines #1 and #10:
Line #1 is where I had the fault confirmed. The same entry ('operation code' and description) appears with the 45,000km service back in 2012. This would seem to confirm that they found the same issue during the warranty period, but either didn't diagnose it properly or chose not to fix it then. My understanding is that would make them still liable.
I'm pretty excited by this, although there's a lot I don't understand in this summary that worries me a little. R/O's? Adv#? Tech maybe is 'technician'? Can anyone decipher?