Nigel, your original unit has an ARF Tx/Rx, if you want to use it with Pilot Aware software you will need to upgrade to the Bridge Tx/Rx. There is little point in getting the ARF back running as it will not communicate to the rest of the PAW users with Bridges. The current software looks for a Bridge Tx/Rx, and will not function without.
Your options are:
1. Roll back to PAW software that is pre-May 2016 IIRC. Use existing Licence until May 2017, when it will stop working.
2. Buy a Bridge Tx/Rx for your old unit, which with its new licence and current software will give you the ability to see ADSB and P3I traffic (No Mode C or s, and No Audio). I took this route for a ground station
3. Remove ARF Tx/Rx, and Install non-PAW ADSB software, with which you can monitor ADSB traffic only
4. Use the Pi for some other purpose.