Problem with text search in Postgresql 10

[Howard News <howardnews@xxxxxxxxxxxxx>]

Hi everyone,

a colleague has restored a backup from an older version of Postgres 
(unknown) to version 10. Everything appears to be working fine with the 
exception of the text search.

If I try:

select to_tsquery('english', 'hello');

I get the error

ERROR:  invalid input syntax for type oid: "english"
LINE 1: select to_tsquery('english','Hello');

I can see from \dF that I have 'pg_catalog.english' installed. It works 
if I do not specify the dictionary using:

select to_tsquery('hello');

Any ideas how to fix this? I have read in older support forums that the 
problem may be related to older contrib methods for ts_query. If this is 
so - how do I remove the contrib decalrations?

Thanks

Howard