genn,
I'm not sure what Rx hardware you're using, but a lot of them have a PLL that requires a nearly even distribution of ones and zeros to work properly. By default, the hardware UARTs send a constant HIGH that will confuse the Rx hardware (and it's not so good for battery life either). So, you want to turn the transmitter's serial port or RF unit OFF until you want to send a packet. Then you send a "preamble" of one or two bytes consisting of either 01010101 or 10101010 to allow the Rx unit's PLL to lock in. Then try sending your data. If your data contains long strings of 1s and 0s, you may need to use Manchester encoding on your data to keep the Rx PLL locked -- hence the popularity of the VirtualWire library (which does the preamble and Manchester [and some error checking too] for you).