Thanks - I had calibrated in Windows early on in the conversion - so I've now reset that to default: but unfortunately the problem remains. Seems strange, as I can't think why there'd even be any feedback at that point in Sega Rally (but never having played the real thing, I have no idea).
I did have to take apart the whole steering assembly, so it's entirely possible the pot centre position is not what it should be - any idea what the correct centre value should be (I guess I could plug it into the drive board and use the 7 segment display). In Model2 emulator calibration reports Min as 9024, centre is about 32700 and Max is 56669.
One other question: what does the small pot on the driveboard do? Mine is currently turned all the way in one direction, but I can't find any mention of it on the online Daytona manuals.