Fixed some big bugs!

There were bugs to do with the write functions and buffers, I had to reduce the block send to 50 bytes as the Arduino Mega was dropping data. Get the associated updated sketch too or you'll have some major issues.
Almost complete now!!

FlashProgrammer/WindowsFormsApp1/Form1.cs

@@ -32,7 +32,7 @@ public Form1()
         private void Form1_Load(object sender, EventArgs e)
         {
             //Find last com port and list it in the control.
-
+            _continue = false;
         }
 
         private void AddToMessageLog(string textToAdd)
@@ -87,6 +87,8 @@ private void Serial_DataReceived(object sender, SerialDataReceivedEventArgs e)
             //try
             //{ 
             string data = serial.ReadLine();        //This line is problematic and maybe should be serial.ReadBytes???
+            if (data.StartsWith("OK"))
+                _continue = true;
             if (data.StartsWith("0x"))
                 this.BeginInvoke(new SetTextDeleg(SerialIn_DataReceived), new object[] { data });
             else
@@ -171,7 +173,7 @@ private void btnWrite_Click(object sender, EventArgs e)
         string WriteFileToFlash(string filename)
         {
             string result = "FAILED!";
-
+            
             //Open the file and read into a byte array
             byte[] dataArray = File.ReadAllBytes(filename);
 
@@ -182,32 +184,37 @@ string WriteFileToFlash(string filename)
             txtMessages.Text += "Writing Flash starting at address 0x0000. Please Wait...";
             int addressCounter = 0;
 
-            //send <X,0,0> then <Z,x,0>   where x is 256 or the number of bytes to write, lastly send <Y> and then the full amount of bytes in sequence  
+            //send <X,0,0> then <Z,x,y>   where x is 256 or the number of bytes to write, lastly send <Y> and then the full amount of bytes in sequence  
             //- Arduino side seems to work just need this side to
             int totalSize = dataArray.Length;
 
             serial.WriteLine("<X," + txtStartAddress.Text + ",0>");
-            serial.WriteLine("<Z," + totalSize.ToString() + ",0>");
+            serial.WriteLine("<Z," + "50," + totalSize.ToString() + ">");
             serial.Write("<Y>");
 
             int offset = 0;
-            int block = 256;
+            int block = 50;
             if (block > totalSize)
                 block = totalSize;
 
             //This part needs checking before allowing a write to Flash!!!!!!!
             while(offset < totalSize)
-            {                
+            {
+                _continue = false;
+                if ((offset + block) > totalSize)
+                    block = totalSize - offset;
                 serial.Write(dataArray, offset, block);
                 if (offset + block > totalSize)
                     offset = totalSize - (offset + block);
                 else
                     offset += block;
-                Application.DoEvents();
+
+                while(_continue == false)
+                    Application.DoEvents();
             }
             //Read the bytes back and compare, using a dump command, ignore and don't read data past where it was written...
 
-            return "Success: Wrote " + (addressCounter + 1).ToString() + " bytes to Flash." + Environment.NewLine;
+            return "Success: Wrote " + totalSize.ToString() + " bytes to Flash." + Environment.NewLine;
         }
 
         private void btnErase_Click(object sender, EventArgs e)