Azure B2C - 401 unauthorised trying to read id_token in code after logging in

I am trying to use the token granted by a secured AAD domain when using my web app

I followed the advice on this link: Retrieve Access Token within a AAD secured Azure Web App

I have managed to get as far as logging in and verifying the ./me URL correctly shows me my token

However when I try and call same token in code I get 401 unauthorised

I have been using the Resource Explorer to configure the additionalLoginParams and have tried to put the app ID as well as the graph URL but nothing has solved the problem

    async public Task<string> GetToken()

    {

        HttpClient _client = new HttpClient();


        string _token = "";

        HttpResponseMessage response = await _client.GetAsync("https://alfreton.azurewebsites.net/.auth/me");
        response.EnsureSuccessStatusCode();
        string responseBody = await response.Content.ReadAsStringAsync();

        ReadUserToken readUserToken = new ReadUserToken();

        readUserToken = JsonConvert.DeserializeObject<ReadUserToken>(responseBody);

        _token = readUserToken.id_token;

        return _token;


    }
}

}

EDIT Following the advice below the code now looks like this but I am still getting an Unauthorized error messsage

    async public Task<string> GetToken()

    {

        HttpClient _client = new HttpClient();


        string _token = "";

        string accessToken = this.Request.Headers["X-MS-TOKEN-AAD-ACCESS-TOKEN"];

        _client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
        HttpResponseMessage response = await _client.GetAsync("https://alfreton.azurewebsites.net/.auth/me");
        response.EnsureSuccessStatusCode();
        string responseBody = await response.Content.ReadAsStringAsync();

        ReadUserToken readUserToken = new ReadUserToken();

        readUserToken = JsonConvert.DeserializeObject<ReadUserToken>(responseBody);

        _token = readUserToken.id_token;


        return _token;


    }

When I read through the headers, I find there is X-MS-TOKEN-AAD-ID-TOKEN - can I use that to get an access token?