Andy
Picking up on Andrea's point of engine type/engine number, have you ascertained what engine type you have (potentially) by reading the stamping on the flywheel rear housing near to the starter motor?
I haven't looked specifically - will see what I can see when I am home later, I've got the starter motor out at the moment being rebuilt so hopefully can find the stamping easily.
From the replies it seems there is no standard for the sump plug location!!