Hi Mike & James,
Mike, the best medels to get hold of are the Panasonic of Pace ones. I have just purchased three from ebay to set up a sky multi room system and all work well with no viewing card.
Have a look here: http://cgi.ebay.co.uk/PANASONIC-TU-DSB30-SKY-DIGIBOX-SATELLITE-RECEIVER_W0QQitemZ130166859066QQihZ003QQcategoryZ96970QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
James, can you borrow a digibox off a neighbour just to check your system, I know the Amstrad is not a very popular model but it may just be a case of repositioning your dish incase it may have been knocked by accident, I know our window cleaner gave ours a bash with his ladder once.
You could also try unpluging the digibox and removing the card, wait for about 30 secs and turn back on, give the card a wipe and reinsert it into the card slot.
Hope this helps
Graham (Ponty)