There are 3 credit reference agencies in the UK
Experian
Equifax
Call Credit
You need to get your file from all three of them. Don't be tempted to just get one or two, get the lot (they usually don't all have the same thing on, believe it or not). This can often be done for free online (print them off) or for £2 by following the links for a "stautory report". If you see things like "£9.50 a month for unlimited access to your file" ignore it, it's not required.
Then on the files it will say if you are financially linked to anyone. You are expecting to see him there. If you do you can apply for a notice of correction to be added. This sounds intimidating because it's jargon, all it really means is you write and say that you broke up a year ago.
You will also be able to see what searches have been done (who has looked at it) and with which credit reference agency or agencies.
So after you've seen the file, got the correction done your next step is to report him for gross misconduct if he has checked your file specifically.
The thing is he might not have checked your specifically, he might have checked his and seen yours because of an outstanding link and decided to tease you a bit. But if he has actually checked yours go get him, he is totally out of order.
It's not just at JCP that this would be gross isconduct, it would be gross misconduct in any job where you have access to otherwise confidential information entrusted to you as part of your role.