In practice, most deeds of sale used by estate agents contain blank spaces for witness's signatures. It is however not a legal requirement that such a contract be witnessed. Only the seller and purchaser need sign the contract and the contract is not invalid if it has not been signed by witnesses.
The underlying idea behind the witnessing of contracts is that a witness can be called in the event of a signatory to a contract disputing his signature thereon. If for example the sellers signature has purportedly been witnessed but the seller denies that it is his signature which appears on the contract then the witness can be called to verify the sellers signature, i.e. the witness can give evidence that the seller signed the contract in his (the witness's) presence. Of course to successfully employ this method of verifying a persons signature the signatory must actually have signed in the presence of the witness. There is accordingly absolutely no point in getting a person to sign as a witness unless the signatory has actually signed the contract in the witness's presence.
Furthermore it goes without saying that one must be able to identify and trace the witness. In many cases where a person disputes his signature to a contract which has been witnessed the witness's identity cannot be determined from his (the witness's) signature and no one can recall who signed as the witness. So, quite often, unless the witness's name and address is printed below their signature the witness's signature is of no value when it comes to verifying a signatory's signature i.e. proving that a particular person signed the contract.
If a party to a contract disputes his signature thereon and the contract has not been witnessed or the witness cannot be traced or it transpires that the party did not actually sign it in the presence of the purported witness then the evidence of forensic handwriting experts is often resorted to when attempting to prove the authenticity of the signature on a contract.