There are two deposits payable.
1. A Booking Deposit to the Estate Agent - which is what you are being asked to pay now. This is paid pre-contract and is paid to show good faith on your part. The amount is not fixed but is typically 2% ish so that EA has ready access to their fees when the deal completes. It is however entirely negotiable.
2. A further non refundable deposit is payable on signature of binding Contracts. This has been typically a total of 10% ( including the booking deposit) when times were good. Nowadays it is very flexible. It can easily be negotiated downwards but a vendor will be suspicious of someone who does not want to pay any deposit. Typically nowadays a total of 4-5% would be most acceptable.
In the event that the sale falls through after Contract and through no fault of the Vendor, they can keep the deposit. This is potent tool in an uncertain market and definitely sorts out the messers.
mf