One Customer, Thirty Days Late: A Cash-Flow Stress Test for Owner-Managers

Most owner-managed businesses do not fail because demand disappears overnight. They fail because a payment that was expected on Friday does not arrive, and the following week contains a payroll run, a VAT bill and a supplier who has already been patient once. Concentration risk is easy to acknowledge and harder to model. The purpose of this article is to make the exposure visible in numbers.

Start with the cash conversion cycle, not the profit and loss account

A £3m turnover business with a healthy gross margin can still be fragile if one customer represents 35% to 45% of invoiced sales. The profit and loss account records revenue when it is earned. The bank account records cash when it arrives. A 30-day delay does not change annual profit, but it changes the order in which obligations fall due.

For stress-testing, use a simple monthly model. Assume annual turnover of £3m, giving average monthly invoiced sales of £250,000. One anchor client accounts for 40% of sales, or £100,000 per month. Standard terms are 30 days. In a normal month, the anchor client pays the previous month's invoice. If payment slips by 30 days, the business carries an additional £100,000 of receivables for a full month. That is the cash hole.

A worked example for a £3m SME

Assume the business holds £150,000 of cash at the start of the month. Monthly payroll is £110,000, paid on the 28th. Trade suppliers are paid on 30-day terms at roughly £140,000 per month. The VAT bill is quarterly at £45,000 and falls due in the stress month. Other customers pay £150,000 in the month, but the anchor client pays nothing because its invoice has slipped.

Cash in: £150,000. Cash out: £110,000 payroll plus £140,000 suppliers plus £45,000 VAT, or £295,000. The month closes with a £145,000 deficit before any overdraft or director's loan. The £150,000 buffer is effectively gone. Payroll may still be met, but only by delaying suppliers, using a credit facility or injecting personal cash. None of those is a strategy; they are consequences.

The second month is usually worse. Suppliers who were paid late tighten terms. The VAT bill has been paid, but the anchor client's payment may still be uncertain. A second 30-day delay creates another £100,000 receivables gap. At that point the business is not managing growth; it is managing survival.

Where the pressure appears first

Payroll is the least flexible obligation. PAYE and National Insurance must be paid, and employees notice immediately if wages are late. A stress test should therefore start with the question: how many payroll runs can the business meet from cash and committed facilities if the anchor client pays nothing? For many £3m businesses, the honest answer is one, perhaps two.

VAT is the second pressure point. VAT collected on sales is not the company's money; it is held on behalf of HMRC until the return is filed and paid. Using VAT cash as working capital is a common informal response to a late payment, but it creates a compliance problem that can escalate quickly. The stress test should treat VAT as a fixed outflow in the month it falls due, not as a buffer.

Supplier terms are the third. A supplier who accepts 45 days once may accept it twice, but not indefinitely. Late payment to suppliers damages credit terms, raises prices and can trigger stop-supply events at the worst moment. The model should show what happens if key suppliers move from 30 days to payment on delivery.

Trigger points to watch

A useful stress test produces trigger points, not just a spreadsheet. Four are particularly relevant for concentrated SMEs.

• Anchor-client concentration above 30% of invoiced sales.
• Cash buffer below six weeks of payroll and fixed overheads.
• Debtor days above 45, or more than 10 days above contractual terms.
• A single customer representing more than 25% of trade receivables.

If two or more triggers are breached, the business should act before the next invoice falls due. Waiting for the late payment to arrive is not risk management; it is reaction.

Credit-check habits that take less than an hour

Concentration risk is not only about payment behaviour. It is also about the customer's own financial health. Before renewing a large contract, check filed accounts at Companies House, look for late filing, changes of directors or charges against assets, and use a credit reference agency for a current score. Ask for trade references from suppliers who have similar terms. Set a credit limit and review it annually. If the customer is much larger than the SME, do not assume that size equals prompt payment; large buyers can be slow precisely because they have leverage.

Negotiation scripts to use before the next delay

The most effective time to discuss payment terms is before an invoice is overdue. Three scripts are worth adapting.

1. The terms reset. 'We value the relationship and want to keep it on a sustainable footing. Our current terms are 30 days, but the recent payment pattern has created a working-capital gap. Can we agree a 14-day payment cycle for the next two quarters, or a 50% deposit on new orders?'

2. The escalation call. 'The invoice dated 3 October is now 30 days past due. I need to confirm the payment date today so I can manage payroll and our own supplier commitments. If there is a query on the invoice, please tell me now so we can resolve it this week.'

3. The supplier conversation. 'We are experiencing a temporary delay from one customer. I want to keep our terms intact rather than pay you late without warning. Can we agree a short payment plan for the next two months, with normal terms resuming in January?'

Make the test routine

A stress test is most useful when it is boring. Update the model monthly, using actual receivables and committed outflows. Keep a one-page summary for the bank and, if relevant, for investors. The objective is not to predict every late payment. It is to know, in advance, which obligations are exposed and which conversations need to happen while there is still room to negotiate.