It seems it was just a matter of changing the Baudrate of the Serial Connection which has provided the solution to my problem. I changed the baudrate from 9600 to 115200 and it seems to be working fine now:

I've accessed the Raspberry Pi through an SSH connection to run the serial test python file.

A simple post splitting process to separate the actual measurement from the letters can be done, so i'm not too worried about that format it is being delivered in. 

*Note, if you are using the serial code files I've uploaded you may need to change the port for the code to 'look at', mine works for ttyUSB0, but this could be different depending on which one you use for yours. A way to check is open the terminal on the the raspbian desktop and run:

ls /dev/tty*

this will then list all the ports beginning with tty. Now plug in your arduino to a USB port and run the command again. A new port will appear at the end of the list, this is what you must reference in your code.