OK, so given my preamble is now extended (it'll be an additional 6ms plus the original 5 bytes worth of the real packet, that 6ms deals with +/-3ms of LO offset drift so receivers still remain locked even if they miss several packets in a row), I think you're suggesting sample RSSI in ultra-fast non AFC mode and look for a set of consecutive RSSI values that are relatively constant for 6ms to indicate a packet might be being transmitted, then I could trigger AFC and try to receive it. If I can measure RSSI fast enough, do the maths and programme into AFC mode within that 6ms timescale then it might work. Thanks for that!
I might need to look for a step change at the beginning given it will need a certain S/N above ambient noise to receive anyway, have you a feel for that value? would 6dB do it?
Mark.
Edit: BTW I've run the test with the receiver put into RX mode while the transmitter is sending preambles now for over 12 hours with a packet drop (two way) of less than 1 in 1000. So that definitely works well.