Just to be on the safe side I ran AdwCleaner and Malwarebytes Anti-malware scans. The AdwCleaner scan showed that the Search Provider on Chrome had been changed to "uk.ask.com". Malwarebytes Anti-malware came up clean.
Should I contact the forum owner to inform them that they probably have a compromised advert on their page? My guess is that this is the most likely cause of the problem.
I include a screenshot of the window which opened: