Client had same over a year ago and responded with a registered letter to the effect:

"We have received an invoice for £XXXX for image
reproduction - after taking professional advice we strongly dispute
this invoice for the following reasons:

1. The images mentioned are not being displayed on our current website [they were removed prior to receiving letter]

2. We have not received any communication from you regarding this
matter other than your invoice, which is against the DMCA (DMCA 1998
Section 512).

3. We have no idea where they came from (a website designer many
years ago used them) and were unaware they were not free to use [important bit!]

4. The Copyright, Designs and Patents Act 1988 (c. forty-eight) states:
"97.—(1) Where in an action for infringement of copyright it is shown
that at the time of the infringement the defendant did not know, and
had no reason to believe, that copyright subsisted in the work to
which the action relates, the plaintiff is not entitled to damages
against him, but without prejudice to any other remedy."

5. Finally:

"It is an offence under Section 40 of the Administration of Justice
Act 1970 and Section 1 of the Malicious Communications Act 1988 to
harass of debtors with a view to obtaining payment including the
issue of letters which convey a threat or false information with
intent to cause distress or anxiety."

We therefore fail to acknowledge your letter or the invoice as legal
or lawful [important bit!]"

notes in [] have been added just now

long thread on it here
WARNING: Getty Images Cracking Down! - SitePoint Forums
you have to register to read it i think