Looks like WSPR will work too. Just tried the same test as last night - namely using a .wav recording of a packet to drive the transmitter - and the PC software happily decodes it.

Next step is to try and squeeze a few more dBm out of the synth. I have a plan to bodge on a 1:1 balun on the outputs of the synth so that both pins are combined (double the power?). Also might try changing Rset to get them to drive more current. Maybe get a whopping 3-4dBm this way?

Finally I need to fit a TX/RX switch (a cheap 5V relay) driven by one of the Arduino pins. This then disconnects the SDR dongle, and connects the transmitter, whenever the PC makes a tone - i.e. we want to transmit.