What if he (and others) decline to be cross examined?
I think you’re getting it the wrong way round mate.
Each party will have submitted a number of witness statements from individual witnesses to advance their case. The opposing side will then either agree the statement because it contains nothing they want to challenge (because there is nothing controversial or it is expedient not to do so) in which case the witness will not be required to attend, but otherwise the assumption is that the witness will be tendered for cross examination, which means attending to give live evidence and being asked questions on the content of their statement, and other matters that are within their knowledge.
It’s not normally obligatory for a witness to attend in this setting (although I think there is the power to compel a witness to do so via the High Court, it is unlikely to be exercised here) and if they don’t then hearsay being permissible the witness statement is admissible as evidence but little weight will be attached to it, as the other side has not had the opportunity to challenge its contents via cross examination.
If they do attend then the other side will have the opportunity to cross examine them. If they don’t answer the question (a tediously common refrain) then the advocate (if they are any good) will simply ask it again until they get an answer, or the tribunal intervenes, and so as a consequence of their failure to answer questions that are put, their evidence will doubtless be viewed as ‘evasive’, ‘not credible’ or worse and will most likely undermine the case of the party they are giving evidence in behalf of.
So, ultimately no-one can be compelled to answer any question, but by avoiding the question, or answering it with a non-sequitur response, or responding with another question etc..all serve to undermine what is contained in their written statement, which is ultimately one of the principal aims of cross examination, although eliciting other answers for wider tactical reasons is another.
As an advocate, it’s the hardest, and usually last skill to master, but at its finest it’s the most effective weapon and what separates the great from the merely good.
And I expect Pannick will be fucking brilliant at it. It usually helps if you’re far smarter than the person you’re asking questions of, which in relation to Masters will unquestionably be the case.