The Concentration Ledger: Stress-Testing Revenue Dependence Before the January Reset

January is rarely generous to UK SMEs. The Christmas orders have been shipped, the January self-assessment payment is looming, and the sales pipeline often looks thinner than it did in October. If a single customer accounts for a large slice of turnover, that seasonal lull can turn into a solvency question very quickly. The answer is not panic. It is a customer-concentration stress test, run before the new year starts, with numbers rather than instincts.

Start With the Revenue Ledger, Not the Gut Feeling

Most owners have a rough idea of who their biggest customer is. Fewer can state, to the nearest percentage point, how much of last year’s gross profit that customer generated. Begin with a simple ledger: list every customer that bought from you in the past twenty-four months, showing net revenue after credit notes and excluding pass-through costs such as disbursements or rechargeable expenses. Then calculate each customer’s share of total revenue.

A common rule of thumb is that any customer above 20 per cent of revenue deserves an amber flag, and any customer above 30 per cent deserves a red one. But revenue alone can mislead. A low-margin distributor taking 25 per cent of turnover may contribute far less to wages and rent than a 12 per cent customer on a 45 per cent gross margin. So run the same calculation on gross profit. If one client dominates both revenue and gross profit, the business has a genuine concentration risk.

For a more rigorous view, the Herfindahl-Hirschman Index — normally associated with competition economics — can be applied to your customer base. Square each customer’s percentage share, sum the results, and compare the total over time. The absolute number matters less than the trend: if the index is rising, your dependence is deepening.

The Notice Period Is the Real Cliff Edge

Concentration only becomes a cash problem when a customer leaves. That is why the contract matters as much as the spreadsheet. Check the notice period on your largest accounts: thirty days, sixty days, ninety days, or termination for convenience with no minimum volume. A client representing 30 per cent of revenue with a thirty-day notice period is a very different risk from one on a twelve-month rolling contract with a ninety-day exit.

Model the timing explicitly. If notice lands on 2 January, when does the final invoice get raised, when is it paid, and which costs remain fixed in the meantime? A useful exercise is a thirteen-week rolling cash-flow forecast with three scenarios: base case, loss of the largest customer, and loss of the top two. Include payroll, rent, VAT payments, corporation tax, loan repayments and the 31 January self-assessment bill. The point is to find the lowest cash point in each scenario and compare it with your available overdraft, cash balance and undrawn facilities.

Take a fictional example. Harlow Precision Ltd turns over £2.4m. Its largest customer accounts for 31 per cent of revenue, or £744,000, at a gross margin of 26 per cent — about £193,000 of gross profit. Fixed overheads are £150,000 a year, and the business holds £90,000 of cash with a £50,000 overdraft. If the customer gives sixty days’ notice in early January, the revenue stops in March. The gross profit lost over the following twelve months is roughly £193,000. Unless replacement work arrives, the business eats through its cash and facility within a few months. That is the cliff edge, and it is visible before the notice arrives.

Apply the Margin Test to Every Major Account

Not all revenue is equal. Before ranking customers, deduct direct costs and client-specific overheads: dedicated account management, travel, bespoke stock, extended payment terms, and the cost of financing receivables. A customer that pays on 90-day terms is effectively borrowing from you, and that borrowing has a price. The resulting contribution margin tells you which accounts actually fund the payroll.

This matters when you build a diversification plan. Replacing £744,000 of low-margin revenue with £500,000 of higher-margin work may be a better outcome than chasing turnover for its own sake. Set diversification targets in gross-profit terms, not just sales terms.

Turn the Stress Test Into a Diversification Plan

A plan needs dates, owners and numbers. A workable format is a one-page concentration action plan reviewed monthly by the board or management team. It should include: a target ceiling for any single customer’s share of gross profit, perhaps 20 per cent by the end of 2027; a named account plan for each customer above 15 per cent; a new-business pipeline target expressed as gross profit, not revenue; and a contract-review schedule covering notice periods, price escalation and minimum volumes.

Consider practical protections too. Credit insurance can cap the loss if a major customer fails, though it will not protect against a customer simply taking its business elsewhere. Invoice finance can release cash tied up in receivables, but it does not replace lost contribution. A contingent plan — which costs would be paused, which hires delayed, which supplier terms renegotiated — should be written down before it is needed.

Finally, put concentration on the board agenda. Lenders, investors and acquirers increasingly ask for customer-concentration disclosure, and a documented stress test is a credible answer. It shows that management understands the risk and has a plan, rather than discovering the problem when the January notice arrives.

A concentration stress test is not pessimism. It is the financial equivalent of checking the fire exits before the party. For UK SMEs heading into a new year, the exercise is quick, inexpensive and considerably less painful than the alternative.