Linux Mint - cannot find auto_explain extension

Hi All,

I'm running Linux Mint 18 Sarah and have installed PostgreSQL 9.5.18 but 
cannot load the auto_explain extension.

postgres=# SELECT * from pg_extension;
-[ RECORD 1 ]--+--------
extname        | plpgsql
extowner       | 10
extnamespace   | 11
extrelocatable | f
extversion     | 1.0
extconfig      |
extcondition   |

postgres=# SELECT * from pg_available_extensions;
-[ RECORD 1 ]-----+-----------------------------
name              | plpgsql
default_version   | 1.0
installed_version | 1.0
comment           | PL/pgSQL procedural language

postgres=#
postgres=# CREATE EXTENSION auto_explain;
ERROR:  could not open extension control file 
"/usr/share/postgresql/9.5/extension/auto_explain.control": No such file 
or directory
postgres=#

Does anyone have any idea what package I need to install to get the 
auto_explain extension please?

TIA

Keith Roberts