The default and hand-off
When an overdraft is defaulted (typically after 3-6 months of persistent excess or non-payment), the account is closed and the debt is marked as in default on your credit file.
The bank's internal collections team may work on the debt for a few months, then it is often sold to a debt purchaser (a company that buys defaulted debt at a discount and pursues it).
Debt purchaser contact
Debt purchasers (Cabot, Lowell, Arrow Global, Intrum, etc) typically contact by letter first, then phone. They ask for full repayment, then negotiate a payment plan.
Debt purchasers can be robust in their approach but are subject to FCA rules on treating customers fairly. Persistent contact should slow if you engage.
You can request that they only contact you in writing.
Court action
Debt purchasers can take court action for a CCJ. This is more likely for larger balances (£1,000+) than smaller ones.
A CCJ appears on your credit file for six years. Enforcement of a CCJ can include bailiffs and attachment of earnings.
If the overdraft is old
Overdraft debts, like other unsecured debts, become statute-barred in England and Wales after six years of no acknowledgement, no payment and no court action. Scotland has a five-year prescription period.
Statute-barred debts still exist but cannot be enforced through court. Debt purchasers occasionally chase statute-barred debts, hoping for a payment that resets the clock. Do not acknowledge or pay a statute-barred debt without advice.