The solicitor acting for the seller issues contracts to the buyers solicitor.
The buyers solicitor read the contracts and issues "requisitions on title" to the sellers solicitor.
The sellers solicitor answers the req's on title to the buyers solicitors satisfaction.
The buyers solicitors ask the buyer to come in and sign contracts and pay a 10% deposit.
At that stage if the buyer has received and returned a letter of loan offer, the solicitor will return contracts to the sellers solicitor. At that stage the buyer is legally bound to the contract.
When the sellers solicitor receives the contract he will ask the seller to come in and sign the contract. Once he signs the contract and returns it to the buyers solicitor there is a legally binding contract in place between the parties.
At that remains to close the sale is to exchange deeds and money, this happens at the closing of the sale.