Rapier,
I assume the event your referring to was the London Motor Show at Earls Court in October 1973. If I were you I'd try and acquire editions of the various UK car magazines that were in circulation at the time (Autocar, Motor, Car etc. etc.) and see if any contain pics of your car. eBay would probably be your best bet for these.
Then try the various photo agencies who were supplying images for these mags. Autocar is still published by Haymarket and their subsidiary, LAT Photographic, will almost certainly have the original negatives if your car was featured. Even if it wasn't, it might be worth an ask anyway...
Hope this helps,
Ben Tyer