Thanks to John Woods and Manuel Bouyer, I've now got something that works.

I needed to use "fdisk -u" to set the BIOS geometry to 1023c/32h/64s,
and manually set the NetBSD partition (which started at block 63) to
start a 0h/1c/0s and all was well.

Pity the Adaptec 1542B doesn't send a "start unit" command (which you
could configure the 1542CF to do) - I have to boot an install floppy to
get my boot disk to spin up :-(

Thanks,
Simon.