Much has been made of the fact that witnesses at the Hutton Inquiry weren't examined under oath. An exception to this seems to be that of the evidence of Professor Hawton: at the start of his transcript is the word (sworn). Presumably he elected to give evidence under oath but was not compelled to. For a person whose testimony was always likely to be more opinion based than fact based he would not be particularly liable to a charge of perjury from what he said.
Another gross failure of the Hutton Inquiry of course was the fact that his Lordship couldn't compel witnesses to attend. I'm flagging this up now because it is easy to say Hutton should have called certain people (for example DCI Alan Young). But it has to be said that it is possible that Hutton did want DCI Young in our example but he, or his bosses said 'no'. In reality I rather suspect that Lord Hutton had a convivial chat with say the Chief Constable of Thames Valley Police and came to a very amicable agreement about who should attend the Inquiry from the police side.
I've just read an interesting fact in Norman Baker's book (page 266) that those making statements in connection with Dr Kelly's death had an opt out at the end of their statement allowing them to decide whether or not they would accept their statement being forwarded to the Inquiry. So looking at the list of witness statements on the Inquiry website and wondering why a statement from an individual hasn't been lodged by Thames Valley Police might be purely because that person didn't want their statement to go forward for whatever reason.