JD Williams Fraud mailing account

Hello,

I recently received a letter to my address from company Lowell claiming I owe money to JD Williams. I quickly searched online and found lots of other people have same problem.
Cut story short I am a victim of identity theft - I left my old address 3 years ago and put a redirect on my post to new address. For three years I didn't receive anything until this week when a letter arrived from Lowell asking to get back to them. Someone at my old property opened a "mailing account" as per my credit reference report and started shopping. I still don't know how this was even possible as all my post was redirected. I tried to get in touch with JD Williams but there process is long winded asking to fill out this form and that. I also noticed someone use my name but DOB on credit report is not correct. I am now in a big shock as what to do?

Please can someone advice as where to start and what to do in this situation?

Thanks