Am i intitled to the deposit back on my holiday if they messed up ?

Okay so i booked a holiday for me and my partner and we agreed with the travel agent that we would pay the final balance on the 20th of july ( as thats when my partner gets paid) so we paid 150deposit (75 each) and an extra fifty at time of booking, soo 200 then we paid 100 to it last month, i recieved a phone cal yesterday tellin me that the balance was due on monday the 20th of june! after arguing with the esate agent she then said that if we wanted to cancel we would not recieve any of our money we paid atall, and we would need to pay another 79 pounds to cancel it!! i dont think this is right as she is the one who messed up the dates adn told us one thing but put in her system another! what do you think i should do ?x