Holiday Lettings and Damage deposits

I have had a nasty experience with this company and wonder if anyone can give me some advice. I booked four days in an apartment in Nice with Holiday Lettings last year. On going through the website booking process, I was given one price but was warned that the owner would need to come back to me with a final price. The "owners did contact me with a revised quote which had increased by nearly 70% on the original quote. However, I noted a comment by the total cost that (with refundable damage deposit of E500.00) which, having booked with HL before and had to pay the damage deposit up front, indicated to me that the E500 was included. I therefore felt that the increased quote was acceptable.
However, it transpires that it was not included. I wrote to HL asking them for the return of the E500 but they said that it had not been charged. I have been toing and froing with emails to HL who refuse to take any responsibility for this. Perhaps I was naive in my booking but having booked with them before believed that my assumptions and the above wording was correct. I now do not know whether I should pursue this or just right it off. Does anyone have any advice for me please?