When One Customer Pays Most of the Bills: A 20-Minute Revenue Safety Check for UK Sole Traders
Most sole traders have a favourite client. They pay on time, they're easy to deal with, and their invoice lands like a warm bath. Then one Tuesday they go quiet. If that thought makes your stomach drop, this check is for you. It isn't about abandoning a good client or chasing shiny new leads. It's a 20-minute revenue concentration check: a calm look at how much of your income rests on one customer, one sector, or one channel — plus two small actions to spread the risk.
First, the reassuring bit
Having one big client is not a failure. In the early years of a sole trader business, it's often how you survive and grow. One strong relationship can teach you more about pricing, scope and delivery than a dozen scattergun leads. The goal isn't zero concentration. The goal is knowing your number, so a surprise doesn't turn into a cashflow crisis.
Think of it like checking the tyre pressure before a long drive. You don't expect a puncture. You just want to spot the slow leak before you're on the hard shoulder.
What to gather (five minutes)
Open your bank statements or accounting software. You want the last 12 completed months of income. If your business is newer, use whatever you have and make a note that it's a partial picture. Use money actually received, not quotes, proposals or invoices that are still sitting unpaid. If you're VAT registered, exclude VAT so you're comparing like with like.
List your customers by name. If one client has several projects or departments, group them as one customer if the relationship is effectively the same. Add a final line called 'everyone else' for the small jobs and one-offs.
The quick maths (ten minutes)
Step one: list your top ten customers, plus 'everyone else'. Step two: add up your total income for the period. Step three: divide each customer's total by the overall total to get a percentage. Step four: group customers by sector or channel too — for example, 'all hospitality clients' or 'all work from one referral partner'. Step five: write down your biggest percentage. That's your concentration number.
A simple example:
| Customer | Income received | Share of total |
|---|---|---|
| Customer A | £38,000 | 51% |
| Customer B | £12,000 | 16% |
| Customer C | £9,000 | 12% |
| Everyone else | £16,000 | 21% |
| Total | £75,000 | 100% |
Your numbers will look different. The point isn't the exact figure — it's the shape of your income.
How to read your result (five minutes)
There are no official rules, but these rough bands are useful:
- No single customer over 20% — fairly balanced. Keep doing what you're doing.
- One customer between 20% and 40% — worth watching, especially if they're also a slow payer.
- One customer between 40% and 60% — high dependence. Make a plan, not a panic.
- One customer over 60% — concentration risk. Put one diversification action near the top of your list.
Do the same for sectors. If 70% of your work comes from one industry, a quiet spell in that industry becomes a quiet spell in your business. That's not a reason to turn work away. It's a reason to know where the next enquiry might come from.
Also look at quality, not just quantity. A top client on a rolling monthly retainer with clear terms is different from a top client who pays late, changes scope weekly and has one contact who might leave. If your biggest customer is also your most unpredictable one, treat that as an amber light.
Warning signs before a cashflow crisis
- Their payments cover your rent, mortgage or regular bills.
- You're nervous about raising prices or saying no to small extras.
- Their work arrives in lumps, with no forward schedule.
- You have no written contract, or one that hasn't been looked at since you started.
- Your pipeline is mostly 'hope they renew'.
- One person at the client holds the whole relationship.
- All your work comes from one sector, one platform or one referral source.
If two or more of those feel familiar, don't spiral. Just choose your next small move.
Two small moves for this month
Action one: protect the relationship. Send your top client a short, friendly email confirming what's included, what isn't, and when invoices are due. Not a confrontation — a routine check-in. Something like: 'I'm planning the next few months and want to make sure I've got your priorities right. Could we book 15 minutes to confirm scope and timings?' That one email can reduce the chance of unpaid extras, surprise gaps and awkward conversations later.
Action two: open a second door. Pick five past clients or quiet contacts and send a specific, low-pressure note. Not 'just checking in'. Try: 'I've got a couple of mornings free next month and I'm taking on one new project. If anything's coming up, happy to have a quick chat.' Or package a small starter service so a new customer can say yes without a big commitment. The aim isn't a full pipeline overnight. It's one extra conversation in a different part of your network.
Put it in the diary
Repeat this check once a quarter. It takes 20 minutes, needs no software upgrade, and gives you a number you can act on. If your biggest customer's share falls, brilliant. If it doesn't, you'll know exactly where to focus — calmly, practically and without waiting for a quiet Tuesday to become a cashflow problem.
