Postgresql – Postgres application tries to use incorrect socket file

I have a local postgres server running (on ubuntu linux). It is listening via a socket file:

$ ls -la /var/run/postgresql/
total 8
drwxrwsrwx  2 postgres postgres  100 2011-04-15 19:06 .
drwxr-xr-x 26 root     root     1100 2011-04-15 19:12 ..
-rw-------  1 postgres postgres    5 2011-04-15 19:06 8.4-main.pid
srwxrwxrwx  1 postgres postgres    0 2011-04-15 19:06 .s.PGSQL.5433
-rw-------  1 postgres postgres   34 2011-04-15 19:06 .s.PGSQL.5433.lock

I can connect to the server fine on the command line:

$ psql -d gis -U rory
psql (8.4.7)
Type "help" for help.

gis=# \q
$ psql -d gis
psql (8.4.7)
Type "help" for help.

 gis=# \q

I am trying to use osm2pgsql, an application from the OpenStreetMap project, which imports data into a pgsql database.

However the error I get is:

$ ./osm2pgsql/osm2pgsql -m -d gis -U rory ../data.osm.bz2 
osm2pgsql SVN version 0.70.5

Connection to database failed: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

Noticably, it is trying to use the socket file .s.PGSQL.5432, which doesn't exist, whereas the actual socket filename is .s.PGSQL.5433, the filenames are almost exactly the same.

Why is it using the wrong filename, and how do I make it use the correct one?