Please let us know which "small independent car repair shop" came to your rescue, this is very useful information!
Having encountered this issue and used some help here, let me return some info relative to 2016 Panda rear door issues
situation door stuck shut, Door was locked, electric motor non operational and child lock was on
1. First thing to do is use spare key to operate locking while car is running sometimes a higher voltage might shift an otherwise stuck motor
2. Check the plug and connection to rear door accessible from front door, remove clean clip on plug , remove boot check for broken wires
3. If no success remove inside panel 2 bolts see other posts here, use cloth for window handle clip then make up a panel clip removal tool something like flat bar ms 2mm thk with 10mm 90 degree bend at bottom to get access behind panel to pull clips
4. once removed there is no access to drive ratchet due to plastic cover however there is a small hole on the door exposing the internal pull cable inserting a long narrow screwdriver beyond this can displace the stuck ratchet and allow access