The 45 days, and telling the manager who was holding my start date

Somebody messaged me about this last week and I gave a worse answer than the question deserved, so I'm putting it here properly.

I didn't pass in March. The studying wasn't the hard part and neither was the shame, and I'd braced for both. It was the email to the manager who had offered me a job with a start date in it, and I sat on that email for four days.

What I sent was three sentences: I didn't pass, my next date is this, I'd still like the position if you can hold it. They held it.

For anyone who's been here: did you tell the employer straight away or wait until you had the new date? And has anyone had an offer pulled over it? I'd rather this thread carried both answers.