In the case of Scicluna v Zippy the Court of Appeal reaffirmed the principle that generally, where there is an agreed List of Issues before an Employment Tribunal, the parties are bound by them. Only exceptionally, it appears, can a Tribunal go behind the agreed list. As ever, when preparing a List of Issues make sure you get it right!!
There are exceptional cases where it may be legitimate for a tribunal not to be bound by the precise terms of an agreed list of issues; but this is not one of them.