Told I owe money back on my benefits after 17 years as a software tester

After 9 weeks of benefits I got a notice saying I must repay some of it. It has been a long 9 weeks. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

Thanks for writing this out. I went through something close to it last year, and it settled down once I kept a simple log of every call and form.