As long ago as 1865 Lord Westbury, LC in Chinnock v. The Marchioness of Ely 4 DE G J&S 638 at 646, observed that:
“As soon as the fact is established of the final mutual assent of the parties to certain terms, and those terms are evidenced by any writing signed by the party to be charged or his agent lawfully authorized, there exist all the materials, which this Court requires, to make a legally binding contract.
But if to a proposal or offer an assent be given subject to a provision as to a contract, then the stipulation as to the contract is a term of the assent, and there is no agreement independent of that stipulation.”
This observation prompted Jessel, MR in Winn v. Bull  7 Ch D 29 at pages 31 and 32, to conclude that:
“where you have a proposal or agreement made in writing expressed to be subject to a formal contract being prepared, it means what it says; it is subject to and is dependent upon a formal contract being prepared. When it is not expressly sta…