Give it a try with this order.

Mockito.when(serviceMock.methodname(params)).thenReturn(responseType);
String json = new Gson().toJson(param);
MockHttpServletRequestBuilder requestBuilder =
   MockMvcRequestBuilders.post(BASE_ENDPOINT)
       .content(json)
       .contentType(MediaType.APPLICATION_JSON)
       .accept(MediaType.APPLICATION_JSON)
       .characterEncoding(CharEncoding.UTF_8);
ResultActions resultActions = this.mockMvc.perform(requestBuilder);
resultActions
   .andExpect(status().isOk())
   .andExpect(
       MockMvcResultMatchers.jsonPath("$.fields").value(param.value())));
verify(serviceMock, times(1)).method(param);

How would this work? I got no idea I'm kinda new to selenium

You could install Fiddler and modify request headers with the CustomRules.js file

To modify the header with Fiddler:

1- Open Fiddler Web Debugger and in the menu toolbar go to Rules->Customize Rules... (shortcut is Ctrl + R). It opens the CustomRules.js file. Fiddler may suggest you to download Fiddler ScriptEditor but you can edit it with any text editor (I use notepad++)

2- Find OnBeforeRequest(oSession: Session) in the script file.

3- You can add a new header by adding oSession.oRequest["NewHeaderName"] = "New header value"; to OnBeforeRequest function. Also, to remove a header, use oSession.oResponse.headers.Remove("HeaderName"); function.

I'm not a software tester, but have been tasked with writing manual User Acceptance test cases for some long customer signup forms for a web application.

Is the starting point that we assume all the form fields are 'there' for each type of form, and that the test cases begin with the checking of the form functionality e.g. input validation etc. Or, should I write a test case to check each form identifier is correct, each form field is present, each drop down is populated etc (I guess when a new build is produced its possible a form element/field could have a error - although unlikely).

If I am going to write a test case for lots of form elements can I use multiple assertions to save time e.g. "Check Identifiers: text1, text2, text3, etc are present and correct." Or should it be one test case for every element of the form. The forms are unlikely to change much over time.

I get the feeling that there are 2 types of test here - one that the form functions correctly, the other the component parts are actually displayed correctly by default.

Thank you.

Give it a try with this order.

Mockito.when(serviceMock.methodname(params)).thenReturn(responseType);
String json = new Gson().toJson(param);
MockHttpServletRequestBuilder requestBuilder =
   MockMvcRequestBuilders.post(BASE_ENDPOINT)
       .content(json)
       .contentType(MediaType.APPLICATION_JSON)
       .accept(MediaType.APPLICATION_JSON)
       .characterEncoding(CharEncoding.UTF_8);
ResultActions resultActions = this.mockMvc.perform(requestBuilder);
resultActions
   .andExpect(status().isOk())
   .andExpect(
       MockMvcResultMatchers.jsonPath("$.fields").value(param.value())));
verify(serviceMock, times(1)).method(param);

Your QA can be a smaller team, and their purpose becomes finding unexpected things. They get to play with the system in ways that a user might. In ways that programmers didn't expect.

Once they find something, you write an automated test so they don't need to repeat it. But you still need them to find those cases.

Not to mention, they'll find things like typos, usability issues with the UI, hard to read colour combinations, etc