All depends on reaction to the budget I guess.
If we’re all the victims of an elaborate deception campaign and the manifesto isn’t trashed (I know, unlikely), or if the budget actually goes down well (I know, even more unlikely) then Starmer will be safe. But a media storm - and I include the BBC in that, i.e. not just headlines in the Mail and Telegraph - and even just a small drop in the polls, and it will break loose.
I don’t doubt the truth of the reporting of Starmer’s ‘save my skin’ briefing campaign. A pre-budget version of getting his retaliation in first.