"2) Contact a credit reference agency: Callcredit, Equifax or Experian provide suggested steps to resolve the situation and prevent it happening again."
Prevent it happening again? What are they going to say, trust any online or government agency with your data!?
These agencies and job sites need vetting now anyway - they've got away with dodgy data sharing activities for years. I haven't uploaded or applied to a job in over 3 years, and despite cancelling the one account I did create all those years ago (Reeds Recruitment), my details are being passed between every dodgy agency in the UK, usually suggesting jobs in the place I was moving away from 3 years ago, 250 miles away from where I'm living now.
Hilariously, I'm always very aware of dodgy check boxes and t&c's, and will always opt out of being shared with other companies, but for some reason, all these agencies seem incapable of telling me where they are getting my details, insisting I must have signed up to them at some point, which is total crap. Ah well.