Fragment method not accessible from Activity

So, I am back to trying to update the EditText content in a Fragment from the Activity. I should be getting the right fragment object from within the viewpager using its position, however the method called, drawSurname(surname), is still giving me 'Cannot resolve' error.

Relevant code from FormActivity;

//initialise pager for swipe screens
        ViewPager pager = (ViewPager) findViewById(R.id.viewPager);
        adapter = new MyPagerAdapter(getSupportFragmentManager());
        pager.setAdapter(adapter);

public void DrawText() {
        // Fetching user details from sqlite
        HashMap<String, String> user = db.getUserDetails();

        if (user.size() != 0) {
            Log.e(TAG, "string surname: " + surname);

            // Displaying the user details on the screen
            Fragment n = adapter.getRegisteredFragment(0);
            n.drawSurname(surname);

        }else{
            Log.e(TAG, "table empty");
        }
    }

public class MyPagerAdapter extends FragmentStatePagerAdapter {
        SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();

        public MyPagerAdapter(FragmentManager fm) {

            super(fm);
        }

        @Override
        public Fragment getItem(int pos) {
            switch(pos) {

                case 0: return FirstFragment.newInstance("@string/form_instruct");
                case 1: return SecondFragment.newInstance("SecondFragment, Instance 1");
                case 2: return ThirdFragment.newInstance("ThirdFragment, Instance 1");
                case 3: return FourthFragment.newInstance("FourthFragment, Instance 1");
                //case 4: return FifthFragment.newInstance("ThirdFragment, Instance 3");
                default: return FirstFragment.newInstance("FirstFragment, Default");
            }
        }

        @Override
        public int getCount() {
            return 4;
        }

        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            Fragment fragment = (Fragment) super.instantiateItem(container, position);
            registeredFragments.put(position, fragment);
            return fragment;
        }

        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            registeredFragments.remove(position);
            super.destroyItem(container, position, object);
        }

        public Fragment getRegisteredFragment(int position) {
            return registeredFragments.get(position);
        }
    }

FirstFragment.java

public class FirstFragment extends android.support.v4.app.Fragment {

    //create variables & Logcat tag
    private static final String TAG = FirstFragment.class.getSimpleName();
    private EditText inputTitle;
    private EditText inputName;
    private EditText inputSurname;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_first, container, false);

        //set inputs for fields
        inputTitle = (EditText) v.findViewById(R.id.titleText);
        inputName = (EditText) v.findViewById(R.id.foreText);
        inputSurname = (EditText) v.findViewById(R.id.surnameText);

        TextView tv = (TextView) v.findViewById(R.id.tvFragFirst);
        tv.setText(getArguments().getString("msg"));

        inputSurname = (EditText) getActivity().findViewById(R.id.surnameText);

        return v;
    }
    public static FirstFragment newInstance(String text) {

        FirstFragment f = new FirstFragment(text);
        Bundle b = new Bundle();
        b.putString("msg", text);

        f.setArguments(b);

        return f;
    }

    public void drawSurname(String x) {

        inputSurname.setText(x);
    }
}

I was sure I'd got the fragment correctly with getRegisteredFragment(), but cannot see why the method wouldn't call otherwise.