Quickly hacked together Java app to read a CSV file and convert old OS Easting and Northing values to Latitude and Longitude. Dependencies: JCoord: http://www.jstott.me.uk/jcoord OpenCSV: http://opencsv.sourceforge.net/

osgb2latlon.java

package com.fiskur.osgb2latlon;

import au.com.bytecode.opencsv.CSVReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import uk.me.jstott.jcoord.LatLng;
import uk.me.jstott.jcoord.OSRef;

/**
 *
 * @author jonathan.fisher
 */
public class osgb2latlon {
    private static final String COMMA = ",";
    private static final String NEW_LINE = "\n";
    private static CSVReader mReader = null;
    private static FileWriter mWriter = null;

    /**
     * Usage: java -jar osgb2latlon.jar somecsvfile.csv
     * @param args
     */
    public static void main(String[] args) {

        if(args == null || args.length == 0){
            System.out.println("No CSV file");
            return;
        }
        
        String filename = args[0];

        if(!filename.toLowerCase().endsWith(".csv")){
            System.out.println("Invalid file format");
            return;
        }

        File csvFile = new File(filename);
        if(!csvFile.exists()){
            System.out.println("File does not exist");
            return;
        }

        String outFilename = filename.substring(0, filename.lastIndexOf(".")) + "_converted.csv";
        File outFile = new File(outFilename);

        if(outFile.exists()){
            outFile.delete();
        }

        try {
            mReader = new CSVReader(new FileReader(csvFile));
            mWriter = new FileWriter(outFile);
            
            String[] colNames = mReader.readNext();
            
            int numCols = colNames.length;
            
            String[] newColNames = new String[numCols];

            int eastingColIndex = 0;
            int northingColIndex = 0;
            boolean foundEasting = false;
            boolean foundNorthing = false;
            
            for(int i = 0 ; i < numCols ; i++){
                String colName = colNames[i];
                newColNames[i] = colName;
                if(colName.toLowerCase().equals("easting")){
                    eastingColIndex = i;
                    foundEasting = true;
                    newColNames[i] = "latitude";
                }
                if(colName.toLowerCase().equals("northing")){
                    northingColIndex = i;
                    foundNorthing = true;
                    newColNames[i] = "longitude";
                }
            }

            if(!foundEasting || !foundNorthing){
                System.out.println("CSV does not contain Easting and Northing columns");
                return;
            }

            writeCSVLine(newColNames);
            
            String [] rowArray;
            while ((rowArray = mReader.readNext()) != null) {
                String easting = rowArray[eastingColIndex];
                String northing = rowArray[northingColIndex];
                OSRef osRef = new OSRef(Double.parseDouble(easting), Double.parseDouble(northing));
                LatLng latLon = osRef.toLatLng();
                latLon.toWGS84();//See http://hannahfry.co.uk/2012/02/01/converting-british-national-grid-to-latitude-and-longitude-ii/
                rowArray[eastingColIndex] = "" + latLon.getLat();
                rowArray[northingColIndex] = "" + latLon.getLng();
                writeCSVLine(rowArray);
            }   
        } catch (FileNotFoundException ex) {
            System.out.println(ex.toString());
            System.exit(-1);
        } catch (IOException ex) {
            System.out.println(ex.toString());
            System.exit(-1);
        } finally{
            try {
                if(mReader != null){
                    mReader.close();
                }
                if(mWriter != null){
                    mWriter.close();
                }
            } catch (IOException ex) {
                System.out.println(ex.toString());
            }
        }
    }
    
    private static void writeCSVLine(String[] data){
        try {
            for(String field : data){
                mWriter.write(field + COMMA);
            }
            mWriter.append(NEW_LINE);
        } catch (IOException ex) {
            System.out.println(ex.toString());
        }
    }
}