"I’ve just been watching The Daily Politics and apparently Parliament agreed some time ago that sensitive material should be withheld."
Depends a little on the timing, and specific details, of this vote, but if it were earlier than the one pertaining to the specific documents the Select Committee wants to see then it's quite likely to be irrelevant. Parliamentary votes aren't binding on the future Parliament, which is free at any time to override what its predecessor did. Hence if it voted that sensitive information should be withheld, then later voted that it should be released, then the most recent vote takes precedence.
Also, look: there are clearly people who voted to Remain who are (a) bitter and (b) clouded by that. But that's no excuse to withhold documents from genuine scrutiny. Parliament -- who, again, I remind you, is the institution to which you voted to give control back -- has a right, which it accorded to itself, to scrutinise these documents. Government can't override or ignore that. I agree that analysis that enters the public domain might risk undermining our negotiating position, although if it's so vulnerable to start with it hardly bodes well, but it's an attitude not unlike that of Kings of old to deny Parliament -- to deny our elected representatives -- the right to scrutinise what the Government is doing. It can be done in a way that allows the necessary and democratic scrutiny to be applied without jeopardising anything.
The alternative is to continue to put absolute and unconditional trust in May, Davis, Fox and Johnson, none of whom have exactly done much to inspire confidence in their ability to put our best interests forward.
It's undemocratic, too. And, yet again, I refer you to the comments of Jacob Rees-Mogg, to which I linked earlier, who can hardly be accused of being a treacherous Remainer trying to ruin our country.