Have you signed the tenancy agreement?

Hello all, I went to view a residential property yesterday to let. Whilst at the property the agent showing me around stated that the total fees that would be charged would be around £300 to £350. She did not give me anything in writing. I really liked the property and as soon as I left called the office and paid the £300 holding deposit (seems a bit high to me), as there were other people viewing the property straight after so I wanted to make sure I got in there fast. I got a call about an hour later from the agent saying that the other people who viewed the property after me had upped the offer by £25 per month and that if I wanted to guarantee the property then she suggested also raising my offer to this amount. I thought about it and really liked the property so agreed. Today I went into the office to pick up my receipt for the £300 holding deposit and the application forms. I asked the lady in the office to clarify the fees and she then told me that they totalled £540!! A whopping £190 more than what was quoted to me whilst viewing the property. I'm not sure that I can afford these fees but now they have taken the excessive holding fee, I am worried that this will not be refunded. They haven't provided me with any terms and conditions and I haven't signed anything. Please help! Thanks