Did you mark your pitch in some way when you went out? If so, you are in the right. If not, I’m afraid this is what can happen.

As normal it's always someone else's fault.

If you didn't leave something on your pitch to indicate it was taken then what has the site staff got to apologise for?

Take responsibility for your own actions.

As a Motorhomer I know you must leave something to mark your pitch when you go out. All arrivals are told to choose an empty pitch - this is what you did and this is what the person who set up on “your” pitch did. Not the staff’s fault. 

PS - you wouldn’t hire a sunbed  for the day and then go off for lunch leaving it empty, would you?!

If you don't leave anything on your pitch when you go out its not the staffs fault that someone else chose that pitch, just like you did. Im sure that the wardens offered you some alternative pitches but obviously not good enough as you left anyway.

While I agree with the sentiments expressed in the replies, it left me wondering if the OP informed the site staff of the pitch he had occupied. Personally I would be somewhat annoyed if the site staff had “ SOLD “ my pitch to someone who had just arrived. It seems a little strange that he had received a call from the site to say his pitch had been re-sold. This implies the site staff knew he had occupied it, so why resell it. 
something not quite right here. 
We were at this site a couple of weeks ago and the site team could not have been more helpful. The trick is to keep them informed. Simples.