The penny finally dropped and I've made the changes to the WMC Guide so that the channel names are identical to those found in Big Screen (which are in turn taken - I presume - from the epg.xml feed I get from our provider).  I also added channel numbers to the Big Screen setup, but I'm not sure if that had any effect - and yes, I asked Big Screen to output channel numbers.

With a few exceptions, the information from the broadcast channels is now great but the information from the Sky channels didn't make it through, and I'm real sure I got the channel names right.  Adding  channel numbers didn't help either.

Here is some info from the XML file:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE tv SYSTEM "xmltv.dtd"[]>
<tv date="20100417150237 +0000" source-info-name="DVB broadcast EPG" generator-info-name="DVBGuide" generator-info-url="http://www.dvbguide.com/">
...

 <channel id="X03F5">
    <display-name>Discovery</display-name>
  </channel>
...
<programme start="20100417063000 +0000" stop="20100417073000 +0000" channel="X03F5">
    <title lang="en">American Chopper</title>
    <desc lang="en">Unique Machine Bike: Sr. takes a trip up to Alaska to research a bike and ends up meeting with Governor Sarah Palin.</desc>
    <category lang="en">Education</category>
    <video>
      <present>yes</present>
    </video>
    <audio>
      <present>yes</present>
    </audio>
  </programme>

Is there any way I can use some of this information to get the program details into the WMC guide?  I'm thinkin of the channel id (  <channel id="X03F5"> ) perhaps.

Also, I'm happy to do some post processing on the XML file if needed - changing channel and display names with string substitutions if that's required.

Thanks.