“i) Questions of some intricacy arise as to the classification of set-offs and the correct approach to be followed when a claim before an arbitrator is met by an argument that there is a set-off available arising under some separate transaction over which the tribunal does not have jurisdiction. Provisionally, I would be minded to think that an arbitrator does or should have jurisdiction to allow a ‘transaction’ set-off, in effect amounting or akin to a defence, to be raised to reduce or extinguish a claim, even though that set-off arises under another contract, outside the tribunal’s jurisdiction: see: Aectra Refining, at pp.1648 and following and Glencore v Agros, at pp.416-417 ... As it seems to me, the investigation and determination of the availability and amount of such a set-off do not involve the arbitrator arrogating to himself a jurisdiction over separate contracts which he does not have (albeit that considerations of issue estoppel may well arise); instead, these steps form part of the process of arriving at a conclusion of whether a defence is properly available in respect of the contract as to which the arbitrator alone has jurisdiction. However, all these observations are provisional only, given that ... such questions do not arise for decision in this matter. ii) Where a decision is called for in respect of a set-off said to arise under a separate contract, then, absent agreement: (1) the point must be properly in issue before the arbitrator; (2) the arbitrator must necessarily investigate the position prevailing in respect of that separate contract; (3) if need be (and unless the arbitrator is proceeding by way of interim award, for example pending a decision on the separate contract by another court or tribunal and with an appropriate reservation of jurisdiction) the arbitrator must make a determination as to the position prevailing in respect of that separate contract; (4) in the light of any such determination, the arbitrator must come to a conclusion as to whether the alleged set-off is indeed available or whether, if not a transaction set-off, it faces a procedural bar, of the nature discussed in the two authorities referred to above”