So this evening I have taken the mono Jack off and replaced it with a TRS Jack. As suggested I have soldered it by connecting tip and sleeve only and left the ring in connected. Switching back the pedal using the app the ‘both’ I can toggle up and down presets. I have switched the automatic calibration back on and I have 0 at heel down and 100 at toe down. It works!
A minor victory for me and now I can get on with the serious business of playing and setting up my preset lists ready for the next gig. Thanks to the Eventide fellas for pointing me in the right direction (and of course for such a fantastic product). Feel free to point others using the boss FV500L in this direction. The boss pedal needs a TRS Jack fitted that the expression pedal end to work as an expression pedal. A bespoke lead is required if you want to run the additional footswitch.