MOSFET analog switch, Transmission Gate, Bilateral switch circuit questions
After the helpful comments above I managed to crack it. So here for the good of humanity is an LTSpice simulation of a MOSFET transmission gate, switching on and off a sine wave sweep from 20Hz to 20KHz over two seconds.
If anyone can find a complimentary P-Channel MOSFET to the BSS83 that has a substrate pin, I would be eternally grateful.
Also if anyone could provide better parameters for the pmos model than the ones shown in the simulation below, please do :)

Note there are some small transients on switching which may cause popping, but this is surprisingly clean switching, better than is likely from a relay. There is some signal attenuation, but again, surprisingly little. I think this is actually viable as a cheap, fairly low distortion dual polarity audio switch. I am super pleased that I decided to stick with this, and post here for help.