Update:
We have been getting rid of the ads one provider at a time to find the culprit. There should now be no ads on the site, and no sign of the unmentionable. If there are any more problems let me know, but there should be no ads in sight.
We're also working on putting measures in place on the ads to prevent them from redirecting the page to another website. This should be in place soon.
We're going to start putting the ads back on bit by bit, apart from the offending provider, and working with them to get the problem resolved once and for all.