This kind of error message:

$ svk pull
Syncing svn+ssh://des3jjm@vega.dur.ac.uk/home/hudson/pg/des3jjm/subversion/docs
Retrieving log information from 249 to 249
    (in cleanup) Svndiff data ends unexpectedly: Unexpected end of svndiff input
Svndiff data ends unexpectedly: Unexpected end of svndiff input
    (in cleanup) Svndiff data ends unexpectedly: Unexpected end of svniff input

Can be caused by:

$ df                                                          
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/engdebian52-root
                                6890048   6560316         0 100% /

So it's really not worth re-mirroring the whole repository...