diff --git a/.github/workflows/default.yml b/.github/workflows/default.yml
index 38d02f76..f6a71de9 100644
--- a/.github/workflows/default.yml
+++ b/.github/workflows/default.yml
@@ -38,7 +38,7 @@ jobs:
           eval "$(conda shell.bash hook)"
           conda activate cmew
           cd climate-model-evaluation-workflow
-          cylc validate .
+          cylc validate . -O metoffice
 
       - name: Run Cylc configuration linter
         run: |
diff --git a/climate-model-evaluation-workflow/flow.cylc b/climate-model-evaluation-workflow/flow.cylc
index 7f5a1765..90995fcd 100644
--- a/climate-model-evaluation-workflow/flow.cylc
+++ b/climate-model-evaluation-workflow/flow.cylc
@@ -1,7 +1,9 @@
 #!jinja2
 # flow.cylc
-{{ assert(SITE != "", "SITE must be set to something other than an empty string") }}
-{{ assert(MODULE_NAME != "", "MODULE_NAME must be set to something other than an empty string") }}
+{{ assert(SITE != "", "SITE must be set to something other than an empty string.
+    Use -O <site> on the command line or set opts to your site through rose edit.") }}
+{{ assert(MODULE_NAME != "", "MODULE_NAME must be set to something other than an empty string.
+    Make sure the opt config file is set through the site.") }}
 
 [scheduler]
     UTC mode = True
diff --git a/climate-model-evaluation-workflow/meta/rose-meta.conf b/climate-model-evaluation-workflow/meta/rose-meta.conf
index 0e3c0f1d..ddf3993c 100644
--- a/climate-model-evaluation-workflow/meta/rose-meta.conf
+++ b/climate-model-evaluation-workflow/meta/rose-meta.conf
@@ -178,8 +178,8 @@ help=If required, this value must be set in a site-specific configuration file
 type=quoted
 
 [template variables=SITE]
-compulsory=true
+compulsory=false
 description=The site at which the Rose suite will run.
 help=This value must be set in a site-specific configuration file located in
     =the 'opt/' directory.
-values="metoffice"
+values="metoffice",""
diff --git a/climate-model-evaluation-workflow/rose-suite.conf b/climate-model-evaluation-workflow/rose-suite.conf
index 1f643053..2716852b 100644
--- a/climate-model-evaluation-workflow/rose-suite.conf
+++ b/climate-model-evaluation-workflow/rose-suite.conf
@@ -1,4 +1,4 @@
-opts=metoffice
+opts=
 
 [template variables]
 DRS_ANA4MIPS="default"