Write Off Unpaid Supplier Invoices

Good afternoon

I have a number of historic, unpaid supplier invoices (ranging between 6 months and 12 months old)

The suppliers have never chased these invoices and I am certain they never will. I intend to raise internal debit notes to write off the invoice values (and pay back the input tax to HMRC that was previously claimed) but am unsure of the following:

1) Do I have a legal obligation to notify the suppliers that these invoices have never been paid?

2) Can I write them back to cost of sales or do they need to go to a "write off" account?

Thanks