Built by THEY.dev, the implementation team specialising in OpenMercato. We build bespoke ERP and operations systems for businesses that run job by job, from first enquiry to paid invoice. Built for the way you actually work, not the way the software wants you to.
An independent THEY.dev build on the open source OpenMercato platform. We are not OpenMercato; we are the team that implements it.
Off the shelf software gets you most of the way, then taxes you per seat for the privilege and never quite matches how you work. So you reconcile the gaps by hand, in spreadsheets and email, on software you will never own.
No two jobs are the same, so the numbers live in spreadsheets that only one person fully understands.
Files, approvals and the current state of a job are scattered across inboxes, with no single place that shows the whole picture.
Per seat pricing punishes you for growing the team, and the moment you stop paying, you lose access to your own way of working.
We do not build for everyone. We build for businesses that run job by job, and we know the detail of these worlds well enough to speak your language from day one. If yours is not named here, the same approach still fits, because the system is built around you either way.
Estimates, orders, drawings and files, deposits and stage payments, all in one thread.
Quotes, subcontractors, CIS, variations and retentions tracked across every live project.
Periodic tenancies, AML and right to rent records, and your whole branch network in one place.
Customers, suppliers, stock, jobs and invoices in one ERP. One record carries each job from first enquiry to paid invoice, costs in against price out, every document attached.
Main contractor: Kingsway Build · PM: you · Started 19 March
On site since 19 March. Two valuations certified, retention held at 5%.
| Site | Stage | Contract value | Margin |
|---|---|---|---|
| Phase 3 fit-out, Kingsway · S-118 | Live | £284,000 | 34% |
| Riverside office refurb · S-121 | Needs action | £142,500 | 27% |
| School extension, Dale Road · S-124 | Tender sent | £96,400 | — |
| Warehouse mezzanine, Unit 9 · S-115 | Invoiced | £58,200 | 31% |
Opened 14 April · Owner: you · Customer: Harbour Wood Developments Ltd
Live since 2 May. Two stage payments received, final invoice due on completion.
| Job | Stage | Price out | Margin |
|---|---|---|---|
| Walnut kitchen, Harbour Wood · 4021 | Live | £48,200 | 34% |
| Oak sideboard, Mill Lane · 4019 | Needs action | £72,400 | 29% |
| Bedroom suite, batch 4, Oakfield · 4024 | Quote sent | £18,950 | — |
| Maple wardrobes, Cedar Court · 4015 | Invoiced | £33,100 | 38% |
Landlord: M. Whitaker · Tenants: 2 · Branch: Parkside
Active since 1 June. Renewal window opens in 47 days, deposit held with TDS.
| Tenancy | Status | Rent pcm | Arrears |
|---|---|---|---|
| 12 Elm Road, Flat 3 · T-7740 | Active | £1,850 | £0 |
| 48 Station Approach · T-7712 | Needs action | £1,425 | £1,425 |
| 7 Orchard Close · T-7731 | Renewal due | £2,100 | £0 |
| 22 Harbour Street, Flat 1 · T-7698 | Vacating | £1,150 | £0 |
Both agreed in writing before we start, so there are no surprises and no open ended bills.
It runs on your own servers. The system and everything in it belong to you, not to us.
Add as many people as you like. The price does not go up because your team grew.
It is yours to keep, host and extend. Take it to another developer whenever you choose.
"On your own servers" sounds like a problem if it means you also have to staff a sysadmin. It doesn't. Pick the level of hands-on you want; the system is yours either way.
We provision and run the infrastructure in your name on Hetzner, AWS or another cloud you choose. You own the account; we keep the lights on.
Runs in your own AWS, Azure or GCP account. Your billing, your IAM, your data residency. We deploy and maintain it under a support agreement.
Your own on-prem hardware or a VPS you already pay for. We deploy, document and hand over. You take it from there, or call us in when you need us.
You don't have to commit to a full build to find out if this is right for you. Start with a free workshop, go deeper with a paid Discovery for £900, and only then decide on the full build at around £20k.
One person who designs and builds the system is the same person you talk to. No account managers, no handovers, no team you never meet. You work directly with the engineer who understands your business.
It is real, production grade software, built to run a business day in and day out, not a prototype dressed up for a demo.
A mature open source base that has been hardened over years. For you that means a lower cost to build and no vendor holding the keys, on a platform that is yours to keep.
And after launch, you are not on your own. Founder-led does not mean one person disappearing the day go-live ends. Backups, security updates, incident response and a clear SLA are part of how we run.
Nightly off-site backups, encrypted at rest. Restore drill documented and rehearsed before go-live, not after the first incident.
Monthly patch window for the OpenMercato base, dependencies and OS. Security advisories tracked, critical fixes shipped out of band.
Business-hour response for normal issues, faster windows for anything that stops you trading. On-call available as an optional retainer.
Full source, infra-as-code, runbooks and an architecture map. You can take it to another developer at any time, with no rebuild needed.
So you know exactly who you are hiring, and what you are buying. A .dev domain can look like an unofficial fork. It isn't. We are the team that implements OpenMercato for UK businesses.
A UK-based software house. openmercato.dev is our practice for ERP builds on the OpenMercato platform. You hire us, we run discovery, deliver the build, migrate your data, train your team, host it if you want us to, and support it after launch.
Start with 80% done. OpenMercato gives you 80% of the ERP out of the box. We build the 20% that makes it yours.
An open source ERP foundation that has been hardened over years. Vendor-neutral. No per-seat licensing. You get the platform's longevity and community without being tied to any one vendor, including us.
An ERP rollout is not a single sprint. It is a sequence of agreed steps with clear exits, so you always know where you stand and what you are signing off.
Interviews, current-state map, pain list. 2 weeks.
How your business actually runs today, written down end-to-end. ~1 week.
What the platform covers as-is, what needs custom work, what stays manual. ~1 week.
The risky parts built first, in your environment, signed off before full build. 1-2 weeks.
The agreed scope built, demoed every fortnight. Typically 6-10 weeks.
From your source system into OpenMercato, with a sign-off pass on a sample. ~2 weeks.
One live session, recorded walkthroughs, written runbook. Cut-over on an agreed date.
4 weeks included after go-live. Optional retainer beyond, or pick a hosting tier.
Off the shelf tools are good at what they do. The question is whether they fit a business that runs job by job, and what you are left holding at the end.
Quick to start, then a monthly bill that never ends and rises every time you add a person.
Built around your business at a fixed price, running on your servers, yours to keep for good.
Powerful and endless, but heavy to set up, costly to run and far more than a focused business needs.
Comparing a specific tool? Read openmercato.dev vs Statii for make-to-order manufacturers.
If your situation is not here, a short call is the quickest way to get a straight answer.
Book a 30 minute callFifteen minutes, no pitch. We will talk through how your business runs today and whether a system you own outright is the right move.
