OK There are free teaching sessions available here on Saturday mornings:

https://guia.barcelona.cat/en/detall/taller-sardanes-a-la-casa-dels-entremesos-a-carrec-de-l-aula-de-sardanes-del-grup-sardanista-iris_99400357299.html

Also nice music and dance sessions at 22.00 on Friday evenings in "placa del rei". Not a Sardana venue but they are very friendly and someone there might know more about the "placa cathedral" Sunday dancers - could even get you an invitation!