When One Invoice Decides the Year: Stress-Testing Client Concentration at London’s Smaller Firms

Dependency is rarely sudden

Most owner-managers across London’s professional services market can name their largest client without pausing. Far fewer can state, without opening the ledger, what proportion of gross profit that relationship generated last quarter — or describe precisely what the firm would look like in the month after it ended.

That gap between instinct and evidence is where concentration risk lives. Dependency seldom arrives through a single dramatic event. It accumulates: a retainer renewed without review, a second engagement added to the same account, a recruitment decision justified by one client’s pipeline. By the time the pattern is obvious, the firm has often rebuilt itself around it.

The 2026 backdrop sharpens the problem. Client buying cycles have lengthened, procurement panels are asking for more evidence and more concessions, and fee pressure is arriving at the same moment as higher employment costs and persistently high office rents. Revenue that looked contracted three years ago behaves more like an option today — and an option exercised by someone else.

First, measure the number you actually mean

Revenue share is the headline metric, and it is a sensible starting point. It is not sufficient. A client representing a fifth of turnover but two-fifths of a partner’s billable time, or three-fifths of the debtor book, is a different order of risk entirely.

A workable concentration test for a firm of ten to a hundred fee earners looks at five measures, taken quarterly:

• Fee income share of the largest client, and of the top two combined.
• Gross margin contribution, after direct costs and any staff dedicated to the account.
• Work in progress and debtor exposure, including any invoice currently past 60 days.
• Share of weighted pipeline for the next two quarters, not merely signed work.
• Share of one partner’s or one team’s capacity, where operational dependency surfaces first.

Set an internal ceiling — many firms settle somewhere between 20% and 30% for a single client — and treat any breach as a management team item rather than a curiosity. A simple concentration index across the whole client base also earns its place: it catches the firm with no dominant client but four relationships that together account for most of the income.

Read the contract as a risk document

Once the numbers are visible, the next step is contractual. Most engagement letters and master service agreements drafted in London are written to win work, not to survive its loss. Read them with a small set of questions in hand:

• What notice must the client give to terminate, and can they terminate for convenience?
• Are there clauses permitting unilateral scope reduction, benchmarking, or periodic price review?
• Does the client hold audit, step-in or reporting rights, or require named individuals to remain on the account?
• How long are payment terms, and is there any right of set-off or withholding?
• On assignment or a change of control, does the contract survive?

Anything under 60 days’ notice deserves attention; 30 days’ notice alongside net-90 payment terms effectively turns the firm into an unsecured lender to its own client. At renewal, notice periods of 90 days, a defined obligation to give notice of material scope reductions, and staged handover fees are reasonable requests that most clients accept without argument.

Audit the workflow, not just the spreadsheet

Concentration shows up in operations long before it appears in the accounts. Warning signs include client-specific systems and templates no other account uses, bespoke reporting built for a single buyer, and a relationship partner who holds the entire history of the engagement. The same test applies on the client’s side: if the firm’s only sponsor is one promotion or one reorganisation away from leaving, the relationship is thinner than it appears.

The practical remedy is a dual-contact rule — at least two people at the firm and two at the client who understand the work and can hold the relationship. It costs almost nothing, it will never feature in a marketing brochure, and it is the single measure most likely to preserve a fee stream when a champion moves on.

Safeguards that fit a smaller firm

Managing concentration does not require a risk function. It requires a short routine and a handful of standing decisions:

1. A one-page quarterly report covering the five measures above, circulated to the management team and minuted.
2. A cash buffer calibrated to loss, not to habit — enough to cover fixed costs for the period it would realistically take to replace the largest client’s contribution, rather than a generic three months.
3. A diversification target by sub-sector, so the firm is not exposed to a single procurement cycle in one industry.
4. An annual scenario test: what happens to payroll, premises and any lender covenants if the top client leaves in month one?
5. An early conversation with the bank and the professional indemnity insurer, since both will price concentration once they see it in the filed accounts.

None of this demands turning away good work. It demands knowing what the work is worth, what it would cost to lose, and how fast the gap could be closed.

The exit-readiness dividend

There is a second audience for this discipline. Any buyer or investor examining a London professional firm will discount heavily for client concentration and will request the figures described above as a matter of course. A firm that can produce them — and show a ceiling it has actually enforced — is worth materially more than one that answers the question with a shrug.

Slower markets make dependency more expensive than it looks. The two-client rule is not a prohibition; it is a prompt to check, every quarter, whether the firm is still the one making decisions about its own future.