Been too busy with real work 
PCBs are now on to Rev 3. I've made a few changes to cut down on switching noise getting through:
- Used lower value resistors in the MPX and pilot divider chains
- Added RC filters after the 4051s
- Pilot generator now works at full 5Vpp level then is attenuated afterwards - this attenuates the noise too
Also some other improvements:
- I found there was too much stereo crosstalk in the original design which is caused by the phase shifts in the output filter putting the MPX and pilot signals out of sync with eachother. So I added a pilot phase adjust control so you can dial it in until its just right. Best way to do this is play a tone on one channel (e.g. left) then set the balance on your receiver to the other (e.g. right) channel (or take the other earbud out). Then adjust the phase until the tone almost disappears. I found I could get >35dB stereo separation like this which is about as good as most tuners.
- Taking the 19kHz SYNC out from the same source as the pilot tone doesn't work, because the filter phase shift puts that out of sync too. So now I take the SYNC out from a dedicated pin on the MCU and have code to shift this in phase by (roughly) the opposite amount.
- Added clamp diodes to the inputs for ESD protection.
- Removed the diode/cap clamp on PIC reset signal - this stopped the PICKit from working.
Data pack is attached with gerbers, hex file and Kicad/MPLAB projects for anyone who wants to build one, plus setup instructions in a PDF.
Also I'll be putting my last three of these (fully built) on ebay soon
