I just noticed that the plug at the phone end of the thingy can be opened without destroying it and inside it there is a small circuit board with a placement labelled "R1". This placement has been shorted out (a single strand from one of the connection wires has been soldered to the other end of it, deliberately) rather than fitting a resistor.
It probably gives the phone the wrong message when connected.
I wonder what value resistor should be fitted? Unfortunately, the mains charger plug cannot be opened without breaking it.
I have the technology if I could find out what the value should be.
Rans6....