invalid field or parameter url when trying to save UserID using Angular

I'm in the process of converting an existing Cllient side Sharepoint 2013 app into Angular.

I'm at the point where i need to save a user to a list, when doing this you need to find out the userID from SP. I would normally use the following code.

function execCrossDomainRequest(selUser) {
//clear any errors
var errModalArea = document.getElementById("modalAnnouncements");
// Remove all nodes from the errAllEvents <DIV> so we have a clean space to write to
while (errModalArea.hasChildNodes()) {
    errModalArea.removeChild(errModalArea.lastChild);
}
var seluser = selUser;
var executor = new SP.RequestExecutor(appweburl.toLowerCase());
// The functions successHandler and errorHandler attend the
//      sucess and error events respectively.
executor.executeAsync({
    url: appweburl + "/_api/SP.AppContextSite(@target)https://waybackassets.bk21.net/siteusers(@v)?@v='" + seluser + "'&@target='" + hostweburl + "'",
    //url: appweburl + "/_api/SP.UserProfiles.PeopleManager/GetPropertiesFor(accountName=@v)?@v='" + seluser + "'",
    //url: appweburl + "_api/SP.UserProfiles.PeopleManager/GetUserProfilePropertyFor(accountName=@v,propertyName='Manager')?@v='" + selManager + "'",
    method: "GET",
    headers: { "Accept": "application/json; odata=verbose" },
    success: successHandler,
    error: errorGetEmployee

});
}

I have adapted this to suit my Angular app...

  // Get the user ID.
function getUserId(loginName) {

    var loginname = loginName;
    alert(loginname);

    var executor = new SP.RequestExecutor();
    // The functions successHandler and errorHandler attend the
    // success and error events respectively.
    executor.executeAsync({
        url: $resource + "/_api/SP.AppContextSite(@target)https://waybackassets.bk21.net/siteusers(@v)?@v='" + loginname + "'&@target='" + $resource + "'", 
       //url: $resource + "/_api/SP.UserProfiles.PeopleManager/GetPropertiesFor(accountName=@v)?@v='" + loginname + "'",
       method: "GET",
       headers: { "Accept": "application/json; odata=verbose" },
       success: successHandler,
       error: errorGetEmployee

     });
}

However I am getting the following error when trying to save.

"invalid field or parameter url"

If I get the function that saves the record a hard coded userID then it works...

Anyone seen this?

S