It is finally working! In hindsight, for a device intended for battery power, the Teensy wasn't a good choice, as it's very difficult to work out exactly what it's doing. This version will send back data (Temp + humidity) to a RasPi server, which logs to a .csv file. I don't intend to pursue this version of the board further, and will be looking at a lower powered implementation using MSP430 or an 8-bit AVR. Also, the form factor can be made smaller using those chips. The pressure sensor does not appear to be working either (tried two boards), which I think may be a problem with the reflowing as I've used these chips before successfully. Working code and boards up on GitHub now, feel free to ask any questions :)