What should a boiler service invoice actually record?

The readings, not the word "serviced". Flue gas analysis figures, what you cleaned, what you tested and anything you flagged as wearing out turn the invoice into a document somebody can file rather than a receipt they lose. It matters most with letting agents, who keep the paperwork that has numbers on it and bin the paperwork that does not.

How should I invoice an out-of-hours callout?

Show the callout, the parts and the labour as three separate lines, with the evening or weekend rate named as such. Bundled into one figure it looks like you charged three hundred pounds for a valve. Broken out, it reads as somebody who came out at nine at night, which is what actually happened and what they were grateful for at the time.

Do my invoice numbers have to be sequential?

They should be unique and consistent. 001, 002, 003 is the easy convention, and it's what makes 'invoice 032' unambiguous on the phone and at tax time. Don't reuse or duplicate numbers; gaps are fine.

When should I send the invoice?

The day the job finishes, from the van ideally. An invoice sent three weeks late quietly tells the customer that paying it late is fine too, and it's the single most fixable cause of slow payment.

Free gas engineer invoice template

A free invoice template for gas engineers, visible in full below, downloadable as a PDF, no signup asked. It has a place for your Gas Safe registration number, which is the one credential a customer can verify in half a minute, and it puts the readings in the line rather than the word "serviced", because a landlord's agent keeps the invoice that has figures on it.

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INVOICE

Your Business Name

Phone · Email

Address

Invoice no.

Date

Due date

Description	Qty	Price
Annual boiler service: strip, clean & test
Flue gas analysis, readings recorded
Supply & fit replacement diverter valve
Evening callout: no heat or hot water
Magnetic filter supplied & fitted
Add your own lines…

Subtotal

VAT (if registered)

TOTAL

Payment details & terms

What a gas engineer's invoice must include

Your business name and contact details (and logo, if you have one)
A unique invoice number, where sequential is fine, gaps are fine, duplicates are not
Invoice date and payment due date
The client's name and the job address
Itemised lines: each piece of work and each material, with its price
The total, and whether VAT (or your local sales tax) is included
How to pay: bank details or a payment link
Payment terms, because "due within 7 days" is a term; silence is a suggestion
Your Gas Safe registration number, because it is the one credential on the invoice a customer can check for themselves in about thirty seconds

Tips from the trade

Put your Gas Safe registration number on every invoice. It is the one credential a customer can check in about thirty seconds, and it does more for the next job than a logo ever will.
Record the readings in the line rather than writing "serviced". Flue gas analysis figures are what make a service invoice a document instead of a receipt, and a landlord's agent will keep the one that has them.
Separate the callout, the part and the labour. A customer who believes a diverter valve cost them the whole total is a customer who rings somebody else next winter.
Invoice emergency work the same evening. You were the person who got their heating back on at nine at night, and by Friday you are one more bill on the worktop.

Common questions

What should a boiler service invoice actually record?: The readings, not the word "serviced". Flue gas analysis figures, what you cleaned, what you tested and anything you flagged as wearing out turn the invoice into a document somebody can file rather than a receipt they lose. It matters most with letting agents, who keep the paperwork that has numbers on it and bin the paperwork that does not.
How should I invoice an out-of-hours callout?: Show the callout, the parts and the labour as three separate lines, with the evening or weekend rate named as such. Bundled into one figure it looks like you charged three hundred pounds for a valve. Broken out, it reads as somebody who came out at nine at night, which is what actually happened and what they were grateful for at the time.
Do my invoice numbers have to be sequential?: They should be unique and consistent. 001, 002, 003 is the easy convention, and it's what makes 'invoice 032' unambiguous on the phone and at tax time. Don't reuse or duplicate numbers; gaps are fine.
When should I send the invoice?: The day the job finishes, from the van ideally. An invoice sent three weeks late quietly tells the customer that paying it late is fine too, and it's the single most fixable cause of slow payment.

Quoting the job before you bill it? Gas Engineer's quote template is the other half of the same paperwork, written for the same trade. Not a gas engineer? The blank UK invoice template works for any trade, in Word, Excel, PDF or Google Docs.

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