Well, to any who are interested, the better diagnostic abilities of the
SunOS4.1.1 netbooter enabled me to see that the problem was with the NFS
mount.  At this point in the boot sequence, my machine seemed to know it
was 'star' but not that it was 'star.mailbox.co.uk', and the nfs export was
to 'star.mailbox.co.uk'.  Slightly mystifying nonetheless, since the SS20
knows that it is both and should have allowed it to mount it regardless,
but changing the nfs export to 'star' solved it.

It would be nice to have better diagnostics in our own netbooter though (I
have now switched back to it).  I am very happy to work on it myself, once
I finish my current NetBSD project - getting ethernet to work on a MacLCIII
(so I can boot my 3/50 off it, which means I can take them both home).

Jules