The problem was caused because there were two copies of lpd running. I've no idea why. Since stopping both and restarting everything works.
EXCEPT...
I can't print from either Firefox or OpenOffice using Computer B. I've used spadmin to set up the new printer but the output does not appear in the spool directory and Firefox does the same. Strange.
Are you using CUPS?
Sounds like the problem we have on FreeBSD with CUPS. There are the system copies of lpr, lpd, lpq and lprm in /usr/bin and the CUPS ones in /usr/local/bin. Default paths use the system ones first. The usual effect is that KDE printing works while OpenOffice printing does not. There are several cures, one of which is to delete (or rename) the versions in /usr/bin.