Thanks for the help folks - the codes don't infact resemble the desired ASCII output !!!
If I take out the ATMEGA328 chip - connect the sensor to pin 1 (TX) - open the serial port at BAUD 38400 - I can see the expected data delivered every 3 seconds as documented - this is the format:
I have no idea how to access this data via C however - I understand the suggestions made and have tried them - the only one I'm not sure about is how to ask for 10bytes and then print them out in the correct format? I guess the 10 byte output for 857 ppm in decimal would be:
32 56 53 55 32 112 112 109 13 10 (last two being carriage return and linefeed) - this is nothing like the output I'm getting?