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"Baffling" - that is one cabinet source's verdict on how No 10 is handling the allegations about the renovations of the Downing Street flat.

at Wednesday's press conference.

And Boris Johnson wouldn't answer the central question in the Commons earlier during a session of Prime Minister's Questions, where he looked more red-faced and furious than I can recall.

The question is a simple one: Who paid the bill for the Downing Street flat makeover at the start?

And for as long as the PM won't answer that, the question will be asked again, and again, and again.

That's not because there's a fascination over whether he and fiancée Carrie Symonds have chosen rattan bedside tables or Persian rugs.

Nor is what matters why they felt it necessary to overhaul the flat, which was - when we visited briefly to film with Theresa May - extremely comfortable and pretty immaculate.

But it matters because there are strict rules about who can give money to politicians, and strict rules that dictate they are meant to tell the rest of us.

Whether you are interested in this particular saga or not, that is a central principle of how politics is meant to work.

And that's why some of Boris Johnson's staff at the time, not just Dominic Cummings, were uneasy with what was going on.

I've been told that there were conversations and concerns about the situation as long ago as February 2020.

The overall chain of events is complicated, and you can remind yourself of what it the story is all about here.

The prime minister said on the record earlier that no rules had been broken.

Yet there are enough question marks for an independent organisation with legal powers to have concluded it is worth a formal investigation.

And the clearest question is, who picked up the tab at the start?

If the PM wants to remove the doubts about what happened, he could publish the information, confront the consequences and then move on.

But, for as long as he won't, his opponents can claim that he has something to hide.