[werk_ag]

Does your TX (green) led blink?

Hi Werk,
No it doesn't, the only sign of life is the red nano LED.
EDIT:Hmm I wonder if the 2N7000 got cooked when I soldered it in.

I forget to say. The TX Led can only blink when the system is powered from the 12V power connector.

It must be doing something though as when I tried to see any coming out the serial port, the IDE always says Cmm Port in use.

Are you using the Arduino IDE Serial Monitor to monitor the output, or any other tool? If any other tool, close it, it my be blocking the serial port.

To see something out from the serial port you must enable Local_TestMode:

Uncoment the line:
#define Local_TestMode

to see debug data for the DHT22 you can also uncomment this line
#define DebugID1

After debuging don't forget to comment those lines again.

I'm curious as to how I would re-upload any software to the Nano now?

Just do it as the first time, compile the code and upload it again. You can do it many many times.

Cheers Werk_AG