If its a 1256 engine it will have 1256 cast in the side of the block. Dout it will be a 1057 engine! Early autos were 1159, but later ones were 1256.
The vehicle details for SXE 579L are:
Date of Liability 01 07 1980
Date of First Registration 09 03 1973
Year of Manufacture 1973
Cylinder Capacity (cc) 1159cc
CO2 Emissions Not Available
Fuel Type PETROL
Export Marker N
Vehicle Status Unlicensed
Vehicle Colour BLUE
As you can see DVLA says 1159.
That chassis number you gave dosent make sense either, check here, http://www.vauxhallviva.com/?page_id=290
Have another look at it. According to that its a 1256, and a 1974. Some photos would help.
Peter