We all know, broadly, what they'll be asking. How did Brown feel about the Iraq War? And did he, as Chancellor, provide enough money for it? In which case, there's a sense that Brown has set himself up for a fall today. Sure, he's not going to admit to either disagreeing with the War, or to underfunding the military. But even the fact that those questions are being asked will be enough to generate some negative copy in the papers tomorrow. And they should have more force attached to them after Geoff Hoon's appearance before the Inquiry in January, and Brown's subsequent emphasis upon the "financial side" of things.
It's clear that Downing Street thinks that Brown would have more to lose by not appearing at Chilcot this side of the election. Perhaps they also think that stories about military underfunding are already priced into public attitudes towards their boss. But if anything sticks to him today, I suspect it will have far more resonance with voters than the recent spate of Westminster-centric scandals. We will find out, shortly, whether there's a BOOM once the ticking stops.