I've only started looking into this properly today as have been busy of late, and I realised one major problem: the transmission needs to be bi-directional because the sensor data needs to sent to the UI PIC from the Field PIC, and the valve-control data needs to be sent from the UI PIC to the Field PIC. This means I would need two sets of RX and TX modules, which would double the cost (from £13 to £26 approx.). And I would also have to use two different bands I'm guessing. Unless there are hybrid RX/TX modules available?