Letting Agent not responding after submitting one-week holding deposit

Hi! I'll be moving to London in September to commence postgraduate studies, and I'll be bringing my family along. We were looking for a place on Rightmove, and found a good 2 bed 2 bath that fit our budget and needs. We did a virtual viewing with the agent from Dexters and happily paid our one-week holding deposit.

We then submitted the necessary documents, but the agent has not been responsive at all. It's almost been one week. We called the office and they said he's not in, and they'll get him to call us back. But he hasn't yet.

What should we do in this case? Will we get our deposit back if this whole thing falls through? Thank you in advance!