diff --git a/src/filters/validationMessageLookup.js b/src/filters/validationMessageLookup.js
index 9ac128da..2abae92a 100644
--- a/src/filters/validationMessageLookup.js
+++ b/src/filters/validationMessageLookup.js
@@ -22,18 +22,18 @@ const validationMessages = {
     fileNameInvalidCharacters: 'The selected file name must not contain any of the following characters: / \\ : * ? " < > |',
     fileNameDoubleExtension: 'The selected file name must not contain two file extensions',
     mimeType: 'The selected file must be a CSV, GeoJSON, GML or GeoPackage file',
-    mimeTypeMalformed: 'The selected file has a malformed mime type'
+    mimeTypeMalformed: 'The selected file has a malformed mime type and is not recognised. Try creating a new version and uploading again, or contact your server administrator for support'
   },
   url: {
     required: 'Enter a URL',
     format: 'Enter a valid URL',
     length: 'The URL must be less than 2048 characters',
-    exists: 'The URL does not exist',
+    exists: 'The URL does not exist. Check the URL you\'ve entered is correct',
     filetype: 'The file referenced by URL must be a CSV, GeoJSON, GML or GeoPackage file',
     size: 'The file referenced by URL must be smaller than 100MB'
   },
   validationResult: {
-    required: 'Unable to contact the API'
+    required: 'Sorry, there is a problem with the service. Try again later'
   },
   name: {
     required: 'Enter your full name'
@@ -47,7 +47,7 @@ const validationMessages = {
   },
   'endpoint-url': {
     required: 'Enter an endpoint URL',
-    format: 'Enter a valid endpoint URL',
+    format: 'Endpoint URL must start with http:// or https://, have no spaces or unusual characters like { } [ ]',
     maxlength: 'The URL must be less than 2048 characters'
   },
   'documentation-url': {
diff --git a/test/unit/dataset-details.test.js b/test/unit/dataset-details.test.js
index d27eebed..1b91c153 100644
--- a/test/unit/dataset-details.test.js
+++ b/test/unit/dataset-details.test.js
@@ -79,7 +79,7 @@ describe(`dataset details View (seed: ${seed})`, () => {
           params,
           fieldId: 'endpoint-url',
           fieldType: 'format',
-          message: 'Enter a valid endpoint URL'
+          message: 'Endpoint URL must start with http:// or https://, have no spaces or unusual characters like { } [ ]'
         })
       )