Clive,
From the description it is all about pre flash as Rob has mentioned. This is discussed a bit in the flash forum. My only contact with Canon was a visiting camera and that only properly worked with a sync lead. I would suggest the only way you will achieve a multiple flash set up is triggering one unit by sync lead and all others by slaves and DO NOT use the pop up. This worked for us using a Canon 300D camera and a mix of various flashguns. Set the camera to manual mode with shutter speed at or below sync speed.
Don
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