I have been told that if I have an accident I may need to pay for (a) the ambulance if needed, and (b) hospital treatment. I was given the example of following an accident at a tennis club, the tennis club's insurance would pay out. Is this true?
I have also been told that Catalans do not have to pay anything  .  This is said to be for foreigners as a way to reduce the cost of so many old folk needing health care.
I did have a motoring accident and it appears my vehicle insurance is paying the ambulance bill and would pay a hospital bill if I get one.
But ...
* what happens if I have an accident at home, or on the beach
* what difference does having a Cat Salut card make
* does having Residencia make any difference?

You are residing in Spain, and need to get Residencia.
Afterwards you can apply for Cat Salut.
As a pensioner? you will not have to pay for this.
Therefore any medical assistance would be paid for, by Cat Salut.