There are many cause of rising damp.
This case is in an Edwardian house on the lounge outer wall. You don’t actually need a moisture meter to identify it here. The plaster and décor are deteriorating. The timber floor in contact with the dampness may be rotting.
The floor needs checking and replacing as necessary. The wall will probably have to be chemically treated and the plaster renewed to a height of about one metre.
Employ reputable and competent contractors who will provide you with a comprehensive insurance backed guarantee.