exception in WMS GetMap

[Nuno Gil]

Hi listers,
I am trying to cascade a layer through Mapserver. When sending a GetMap request:
http://iceds.ge.ucl.ac.uk/cgi-bin/wms?map=wmsjisc.map&VERSION=1.1.1&REQUEST=GetMap&SRS=EPSG:27700&BBOX=427445.7381027595,262172.15873517306,436304.2618972405,268077.84126482694&WIDTH=600&HEIGHT=400&LAYERS=Raster50K&STYLES=&FORMAT=image/gif&BGCOLOR=0xffffff&TRANSPARENT=FALSE&EXCEPTIONS=application/vnd.ogc.se_xml&
I get this exception error:

<?xml version='1.0' encoding="ISO-8859-1" standalone="no" ?>
<!DOCTYPE ServiceExceptionReport SYSTEM
"http://schemas.opengeospatial.net/wms/1.1.1/exception_1_1_1.dtd">
<ServiceExceptionReport version="1.1.1">
<ServiceException>
msDrawMap(): WMS connection error. Failed to draw WMS layer named
&apos;Raster50K&apos;. This most likely happened because the remote
WMS server returned an invalid image, and XML exception or another
unexpected result in response to the GetMap request. Also check and
make sure that the layer&apos;s connection URL is valid.
msDrawRaster(): Image handling error. Unrecognized or unsupported image format
drawEPP(): Image handling error. 1112197850196690.img.tmp is not an EPPL file.
</ServiceException>
</ServiceExceptionReport>

Can somebody help me understanding it? Is there anyway of checking the
request that Mapserver is sending to the cascaded server?

Regards,
Nuno