getControl on control in skin XML? - Kodi for PC IOS Football Addons IPTV M3u Sports

Hi,
Trying to write a script to modify some controls in a windows defined in the skin (ultimately some sliders, in the manner of this post)

Have a window defined like this:

Code:
<window id="6660">

    <onload>runscript(special://skin/extras/scripts/timeshiftprogress.py)</onload>

    <controls>

        <control type="label" id="6661">

            <top>134</top>

            <left>700</left>

            <width>50%</width>

            <height>60</height>

            <label>XXXX</label>

            <shadowcolor>black</shadowcolor>

        </control>

          ......

The script contains:

Code:

import xbmc

import xbmcgui

w = xbmcgui.WindowDialog(6660) c = w.getControl(6661) c.setLabel('hello')

The window comes up and shows the label, the script runs, but Kodi reports
EXCEPTION: Non-Existent Control 6661 in the log. Any existing control id in the XML behaves the same.

Should I be able to do this? I have read hints elsewhere that only script-created controls can be got with getControl; is this the case? (I will try it, anyway)

Curiously, if I try to get the window ID with xbmcgui.Window(6660) it reports a nonexistent window. Not sure if this is a clue (the window actually is a dialog, afaik – it is DialogSeekBar.xml of estuary)