Thanks very much for these detailed responses chaps.
The idea that one of the boxes might take off when the other transmits is intruiging, but remember this happens both ways round, ie 430 breaking into MX170 or vice versa. Very interesting point about the headphone not being muted though. I wonder if a lack of equalising resistor is a factor. However you can listen to both radios simultaneously with similar audio levels - it's only when you tx on one of them that the breakthrough occurs.
I'm also wondering about the Sigtronics box. They publish a sidetone mod for radios that don't supply sidetone. I'll check if this is fitted. Sidetone on the MX170 is much less than the 430.
I'm still clinging to the hope that this is a simple wiring error and that I won't have to buy yet more new boxes!
Thanks again