Thanks, that makes sense now. Looks like Q4 is also switching the battery supply to the oscillator on when an audio signal is detected. Would it be correct to say that the audio amplification of Q1 and Q2 is only to feed the audio signal detector to ensure it is triggered even at low signal levels? I don't see that any audio from this section is being fed into the oscillator.
I just didn't think a 2 transistor transmitter would be useable in a commercial audio product and therefore assumed they were using some kind of special stabilising circuit - seems not.