[twill] Redirected HTTPS page content not available

[argetek senvx]

Hi,

Using twill, a resource I am accessing requires a redirection for
authentication to another web server (a Shibboleth IdP FWIW).  The
redirection seems to go fine but showforms shows nothing (there should
be a login form), the info output also lists the title as null (even
though the login page sets a TITLE) and save_html results in an empty
file.  A twill-sh session shown below:

>> go http://www.ndrb.org.uk/buscador/BuscarAvanzadoCU/MostrarResultadosImagenesPrepararRetornoDetalleImagenes.do?idioma=en&buscadorContenido=.*&pagina=1&formato=&recurso=&procesoCognitivo=&contexto=&edad=&autor=&diaPublicacion=&mesPublicacion=&anyoPublicacion=&c_s_secuencia=&valoracion=&areaCurricular=&identificadorODE=es-sx_20100205_3_9111612&tipoVisualizacion=&nodoDestino=&enlaceComunidadesSeleccionadas=&tipoBusqueda=03&enlaceTodoAgrega=&tipoLayoutBuscador=BUSCADOR&idTesauro=&nomTesauros=&idTesauroSugerencia=&nivelAgregacion=&destinatarios=&keyword=art&numeroResultados=
==> at https://identity.siriusit.co.uk:443/idp/Authn/UserPassword
current page: https://identity.siriusit.co.uk:443/idp/Authn/UserPassword
>> showforms
current page: https://identity.siriusit.co.uk:443/idp/Authn/UserPassword
>> info

Page information:
        URL: https://identity.siriusit.co.uk:443/idp/Authn/UserPassword
        HTTP code: 200
        Content type: text/html (HTML)
        Page title: Null

current page: https://identity.siriusit.co.uk:443/idp/Authn/UserPassword

I've also tried setting the agent like so:

>> agent 'Mozilla/5.0 (X11; U; Linux i686; en-GB; rv:1.9.0.14) Gecko/2009091010 Iceweasel/3.0.6 (Debian-3.0.6-3)'

No joy.

Any ideas?

--
Regards,

     ~ argetek