Left it running; it ended with:

$ bzr clone git+ssh://<email address hidden>/git/gtk+ tryagain
bzr: ERROR: The branch git+ssh://<email address hidden>/git/gtk+ has no revision None.
$

And this was in ~/.bzr.log:

Fri 2011-04-15 00:59:37 +1000
0.085  bazaar version: 2.3.1
0.085  bzr arguments: [u'--version']
0.131  looking for plugins in /home/andrew/.bazaar/plugins
0.131  looking for plugins in /usr/lib/pymodules/python2.6/bzrlib/plugins
0.147  looking for plugins in /usr/lib/python2.6/dist-packages/bzrlib/plugins
0.278  encoding stdout as sys.stdout encoding 'UTF-8'
0.298  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
0.299  return code 0
5080.205  Auto-packing repository GCRepositoryPackCollection(CHKInventoryRepository('file:///home/andrew/vcs/gtk%2B/.bzr/repository/')), which has 11 pack files, containing 20000 revisions. Packing 10 files into 1 affecting 10000 revisions
5080.404  repacking 10000 revisions
5090.721  repacking 10000 inventories
5097.277  repacking chk: 9972 id_to_entry roots, 319 p_id_map roots, 59500 total keys
5147.501  repacking 55027 texts
5401.151  repacking 0 signatures
5413.738  Auto-packing repository GCRepositoryPackCollection(CHKInventoryRepository('file:///home/andrew/vcs/gtk%2B/.bzr/repository/')) completed
6262.603  Auto-packing repository GCRepositoryPackCollection(CHKInventoryRepository('file:///home/andrew/vcs/gtk%2B/.bzr/repository/')), which has 12 pack files, containing 30000 revisions. Packing 10 files into 1 affecting 10000 revisions
6262.805  repacking 10000 revisions
6271.347  repacking 10000 inventories
6278.311  repacking chk: 10000 id_to_entry roots, 619 p_id_map roots, 43476 total keys
6319.078  repacking 35011 texts
6494.092  repacking 0 signatures
6503.293  Auto-packing repository GCRepositoryPackCollection(CHKInventoryRepository('file:///home/andrew/vcs/gtk%2B/.bzr/repository/')) completed
6954.518  creating branch <bzrlib.branch.BzrBranchFormat7 object at 0x7f524a96f590> in file:///home/andrew/vcs/gtk%2B/tryagain/
6958.981  Transferred: 0kB (0.0kB/s r:0kB w:0kB)
6958.991  Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/bzrlib/commands.py", line 926, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/pymodules/python2.6/bzrlib/commands.py", line 1126, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/pymodules/python2.6/bzrlib/commands.py", line 691, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/pymodules/python2.6/bzrlib/commands.py", line 713, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/pymodules/python2.6/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/pymodules/python2.6/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/pymodules/python2.6/bzrlib/builtins.py", line 1233, in run
    raise errors.BzrCommandError(msg)
BzrCommandError: The branch git+ssh://<email address hidden>/git/gtk+ has no revision None.

6958.991  return code 3

I did do this under a (freshly created) shared repository and there are 725 MB of packs there, so presumably the I can try again / resume. Should I? Or is there another diagnostic you'd like me to have a go with?

AfC