Trouble accessing customiser

I am trying to access the customiser for our site: stgregorysfoundation.org.uk/test/ and have got this error message several times: Fatal error: Allowed memory size of 41943040 bytes exhausted (tried to allocate 19456 bytes) in /home/fhlinux201/s/stgregorysfoundation.org.uk/user/htdocs/test/wp-admin/includes/template.php on line 290

Forgive me if it is obvious what I should do, but I’m not sure why I am getting the message or what I can do about it. Is the memory the message refers to the memory provided by our host meaning I need to upgrade our hosting package, or is it at the WordPress end?