When the same thing happened to me I sent them a cease and desist instruction and a time limit on removing my images, in order to give them the opportunity to remove the images. When they tried to fob me off telling me the campaign would end soon and would remove them then I considered their use a wilful breach and not a mistake that an apology and compliance could have smoothed over. I calculated the cost of what I wanted for the usage, including the subsequent use, by working out the earliest date the images may have been used, and extrapolating to the date they indicated they would be removing them, and then applying Alamy's usage rates to that time period and multiplying by the number of images used. It took several months, following a few emails to the person I identified as responsible for the site in question, and later their company lawyers who tried to tell me they had no contract with me therefore they had nothing to pay. I followed that up by advising them to instruct their clients to pay as their lawyers had now admitted wilful use without a license. After a couple of months they paid in full, which probably saved them quite a bit of money as had I taken it to court (which I was prepared to do) it would have cost them a lot more, and the bad press would have seriously damaged them.
I would advise that you similarly stick to your guns and follow up. It was fairly straight forward for me to work out costs as it was just web usage, as yours includes press releases I would advise having a good look at the advice here:
http://www.londonfreelance.org/feesg...ll&subsubs=All and above all don't be bullied by them - the company that used mine is a multi-billion dollar computer company with offices all over the world, and the firm of lawyers they hired to deal with me had 160+ partners, which was clearly intended to intimidate. However the law is on your side and you just have to have faith in it and yourself.
Be polite, give them the opportunity to put it right by removing them from their website, and invoice for the use to date with a fair rate based on guideline amounts (as I said, I chose Alamy's rates, but it may be better for you to use the NUJ ones). Then if they don't comply you can take it as a wilful misuse and have the right to sue. If you need legal guidance then there are lawyers with expertise in this area that can help you, eg.
http://ipprotection.net/index.php
Good luck and I hope you get it resolved without too much stress,