Leave a note on their windscreen explaining the situation, saying you entered into a lease in good faith believing the space was unused and ask them to call you so you can understand what the story is as the landlord & agent say it should be unoccupied.
Get their side of the story and the go back to the landlord. I wouldn't pay anymore rent until it's sorted.
They're probably having a bun fight over something and you're in the middle of it so I'd keep an eye out for an alternative spot too