The Convention is concerned to promote the efficacious conduct of proceedings and to avoid multiplicity of closely related proceedings in different contracting states. It would be surprising and unfortunate if, having decided that the contract is null and void, the same court cannot proceed to decide on the restitutionary consequences following directly from this. What matters is not whether the consequential relief is classified by English or Scottish law or the law of some other contracting state as part of its national law of contract or part of its national law of restitution. What matters is that, however labelled, the relief is no more than part of the effective determination of a dispute relating to a contract. It is one facet of a single dispute.
Contracts are consensual arrangements.
Part payments made in advance in purported performance of a contractual obligation are likewise made and accepted on an agreed, consensual basis. They are made on the terms of that contract, and they are explicable only by reference to those terms. This remains so, even if subsequently the contract if found to be void in law. The next step in this line of examples is to assume that P reverses the order in which he makes his two claims in the proceedings. D is constantly prevaricating, sometimes saying there is a contract, sometimes not.