diff --git a/app/main/forms.py b/app/main/forms.py
index 92333c47d..cd4617f31 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -637,33 +637,6 @@ class RegisterUserFromOrgInviteForm(StripWhitespaceForm):
     auth_type = HiddenField('auth_type', validators=[DataRequired()])
 
 
-def extend_params(params, extensions):
-    items = None
-    param_items = len(params['items']) if 'items' in params else 0
-
-    # split items off from params to make it a pure dict
-    if 'items' in extensions:
-        items = extensions['items']
-        del extensions['items']
-
-    # merge dicts
-    merge_jsonlike(params, extensions)
-
-    # tidy up
-    extensions['items'] = items
-
-    # merge items
-    if items:
-        if 'items' not in params:
-            params['items'] = extensions['items']
-        else:
-            for idx, _item in enumerate(extensions['items']):
-                if idx >= param_items:
-                    params['items'].append(extensions['items'][idx])
-                else:
-                    params['items'][idx].update(extensions['items'][idx])
-
-
 def govuk_checkbox_field_widget(self, field, param_extensions=None, **kwargs):
 
     # error messages
@@ -695,11 +668,11 @@ def govuk_checkbox_field_widget(self, field, param_extensions=None, **kwargs):
 
     # extend default params with any sent in during instantiation
     if self.param_extensions:
-        extend_params(params, self.param_extensions)
+        merge_jsonlike(params, self.param_extensions)
 
     # add any sent in though use in templates
     if param_extensions:
-        extend_params(params, param_extensions)
+        merge_jsonlike(params, param_extensions)
 
     return Markup(
         render_template('forms/fields/checkboxes/macro.njk', params=params))
@@ -751,11 +724,11 @@ def govuk_checkboxes_field_widget(self, field, wrap_in_collapsible=False, param_
 
     # extend default params with any sent in during instantiation
     if self.param_extensions:
-        extend_params(params, self.param_extensions)
+        merge_jsonlike(params, self.param_extensions)
 
     # add any sent in though use in templates
     if param_extensions:
-        extend_params(params, param_extensions)
+        merge_jsonlike(params, param_extensions)
 
     if wrap_in_collapsible:
 
@@ -805,11 +778,11 @@ def govuk_radios_field_widget(self, field, param_extensions=None, **kwargs):
 
     # extend default params with any sent in during instantiation
     if self.param_extensions:
-        extend_params(params, self.param_extensions)
+        merge_jsonlike(params, self.param_extensions)
 
     # add any sent in though use in templates
     if param_extensions:
-        extend_params(params, param_extensions)
+        merge_jsonlike(params, param_extensions)
 
     return Markup(
         render_template('components/radios/template.njk', params=params))
diff --git a/tests/app/test_utils.py b/tests/app/test_utils.py
index b869e9a6a..b198cf1b0 100644
--- a/tests/app/test_utils.py
+++ b/tests/app/test_utils.py
@@ -616,6 +616,8 @@ def test_get_sample_template_returns_template(template_type):
     ({"a": {"b": "c"}}, {"a": {"e": "f"}}, {"a": {"b": "c", "e": "f"}}),
     # same key in both dicts, value is a string, destination supercedes source:
     ({"a": "b"}, {"a": "c"}, {"a": "c"}),
+    # nested dict added to new key of dict, additive behaviour:
+    ({"a": "b"}, {"c": {"d": "e"}}, {"a": "b", "c": {"d": "e"}}),
     # lists with same length but different items, destination supercedes source:
     (["b", "c", "d"], ["b", "e", "f"], ["b", "e", "f"]),
     # lists in dicts behave as top level lists
@@ -626,8 +628,14 @@ def test_get_sample_template_returns_template(template_type):
     ([{"b": "c"}], [{"b": "c"}], [{"b": "c"}]),
     # if dicts in lists have different values, they are not merged
     ([{"b": "c"}], [{"b": "e"}], [{"b": "e"}]),
+    # if nested dicts in lists have different keys, additive behaviour
+    ([{"b": "c"}], [{"d": {"e": "f"}}], [{"b": "c", "d": {"e": "f"}}]),
     # merge a dict with a null object returns that dict (does not work the other way round)
-    ({"a": {"b": "c"}}, None, {"a": {"b": "c"}})
+    ({"a": {"b": "c"}}, None, {"a": {"b": "c"}}),
+    # double nested dicts, new adds new Boolean key: value, additive behaviour
+    ({"a": {"b": {"c": "d"}}}, {"a": {"b": {"e": True}}}, {"a": {"b": {"c": "d", "e": True}}}),
+    # double nested dicts, both have same key, different values, destination supercedes source
+    ({"a": {"b": {"c": "d"}}}, {"a": {"b": {"c": "e"}}}, {"a": {"b": {"c": "e"}}})
 ])
 def test_merge_jsonlike_merges_jsonlike_objects_correctly(source_object, destination_object, expected_result):
     merge_jsonlike(source_object, destination_object)