Accept with alacrity
Q: Do I have to accept the post immediately it is offered?
A: Assuming you have no need to discuss terms, such as salary or starting date, I don't see why you should not verbally accept straight away. If you have any doubts that have arisen during the process, this is the time to raise them, and if they cannot be dispelled, then you would no doubt decline the post there and then.
If you have questions for John Howson, email them to email@example.com