http://www.whatdotheyknow.com/request/advice_to_the_internet_watch_fou They just responded, rejecting my request under s17(4):

In accordance with the Act, this letter represents a Refusal Notice for this particular request under Section 17(4).

Section 17(4) of the Act provides:

(4) A public authority is not obliged to make a statement under subsection (1)(c) or (3) if, or to the extent that, the statement would involve the disclosure of information which would itself be exempt information.

The Metropolitan Police Service can neither confirm nor deny that it holds the information you requested as the duty in Section 1(1)(a) of the Freedom of Information Act 2000 does not apply, by virtue of the following exemption:

S30(3) Investigations

The MPS will not disclose whether it has carried out an investigation, or whether an investigation is taking place, unless that information has already been placed in the public domain, through channels such as the media or the court process. Nor will we confirm whether or not correspondence has been received by a force from a third party.

Any disclosure under Freedom of Information is a release of information to the world in general and not an individual applicant. Therefore, simply confirming or not that such information were held would reveal which force has and hasn't had contact with the IWF.

I don't suppose anyone has any evidence that the IWF definitely contacted a specific police force, do they? :o)