If both sides are sensible, this won’t happen. That doesn’t mean that you’ll always agree what’s relevant – it’s just that it’s rarely worth falling out over. If there’s a really good reason not to want the tribunal to see it at all – for example, because it’s privileged – and it’s important enough, you may need to ask the tribunal to conduct a case management discussion so that an employment judge can rule on the question. But otherwise – if the other side wants to put something in, let them. If you don’t think it’s relevant, save your breath for explaining to the tribunal why they should ignore it.
Of course, ‘if both sides are sensible’ is quite a big if. The other side may just refuse to agree to a bundle that includes all the documents you want to rely on.
But if this happens, don’t waste time quarreling about it. Compile a bundle of all the documents you want, plus all the documents the other side wants. Send them a draft index page, and invite them to tell you if you’ve missed anything they want included. Tell them you propose to bring sufficient copies of your bundle to the hearing, whether they agree or not. If they want to produce a more limited rival bundle, that’s a matter for them. As long as yours doesn’t leave out anything they want in, yours will be more complete. As long as it’s not too huge, that will probably be enough to persuade the tribunal to use it.
Slightly different considerations may arise in the Employment Appeal Tribunal, where there’s a rule that the bundle shouldn’t normally exceed 100 pages. If you think you need more than 100 pages, you’ll need to write to the Registrar explaining why, and asking for permission to lodge a longer bundle. If the other side disagrees, they will probably write in to object. The Registrar or a judge will decide. (See paragraphs 4 and 6 of the EAT Practice Direction.)
If your version of the bundle comes in under 100 pages, but the other side still objects to some of what you want to include, just lodge your version. Explain in the covering letter that you haven’t been able to agree it with the other side, but it includes all the documents they say they want to rely on, plus some others that you consider relevant and they object to.