You can collect the fixed monthly maintenance charge and the variable call-outs, remedials and parts under one direct debit mandate, and the customer never has to sign again when the total changes. Each collection at a new amount needs advance notice, and your invoice can be that notice. For most contractors the simplest pattern is one combined collection a month: this month's fixed charge plus last month's reactive work.
In this article
The short version
- One mandate can collect different amounts on regular or irregular dates, so a customer can pay £420 one month and £923 the next without re-signing.
- Query any paper mandate returned with a handwritten limit, because a capped instruction won't cover the variable half of the contract.
- Each collection at a new amount needs notice of 10 working days under the Bacs default, and invoicing on the 1st for collection on the 20th clears it.
- The contract should set out what the fixed charge includes, the call-out and parts rates, the collection day and a pre-approval limit for reactive work.
- If a customer cancels the mandate, the contract still stands and the charges can be collected by card or bank transfer until a new mandate is signed.
Can one mandate cover the fixed charge and the call-outs?
Yes. Most fire, security and maintenance contracts have a fixed part (planned servicing and, for alarms, monitoring) and a variable part (call-outs, out-of-hours attendance, detector heads, batteries, remedials found on a visit). Plenty of firms run the fixed part on direct debit and email invoices for everything else, then spend the month chasing the variable half.
A mandate isn't tied to one amount. Under the Direct Debit scheme you can collect different amounts on regular or irregular dates from the same instruction, so a customer can pay £420 one month and £923 the next without signing anything new. Watch for a paper mandate that comes back with a handwritten limit on it. A capped instruction won't cover the variable half of the contract, so query it before you rely on it.
You can run collections in two ways under that one mandate. Combined means one collection a month for the current fixed charge plus the previous month's variable work. Separate means the fixed charge on a set date and variable work collected on its own date. Combined gives the customer one invoice, one notice and one debit a month. Separate suits customers who want to approve every call-out line before it's debited.
An example monthly schedule
Take a firm maintaining a fire alarm and an intruder alarm across a customer's two sites. The fixed charge is £420 a month including VAT, call-outs are £145, out-of-hours attendance is an extra £95, and parts are billed at the agreed rate. The firm invoices on the first working day of the month and collects on the 20th, or the next working day.
| Month | Invoice sent | Collected | Fixed | Previous month's variable work | Total |
|---|---|---|---|---|---|
| October 2026 | Thu 1 Oct | Tue 20 Oct | £420.00 | 1 call-out £145.00, parts £62.50 | £627.50 |
| November 2026 | Mon 2 Nov | Fri 20 Nov | £420.00 | None | £420.00 |
| December 2026 | Tue 1 Dec | Mon 21 Dec | £420.00 | 2 call-outs £290.00, out-of-hours £95.00, parts £118.00 | £923.00 |
Every collection whose amount differs from the last needs notice of the amount and date: 10 working days under the Bacs default, or a shorter period if one was agreed with the customer at set-up. Invoicing on the 1st and collecting on the 20th clears the 10 working day default with a few days to spare, which absorbs a late job sheet or a query without moving the collection date.
If the customer queries the December parts line and you agree to drop it, send a corrected invoice for £805.00 before the 21st. Never collect more than the figure you notified.
What to set up in the contract and on the invoice
The mandate only authorises collections. What the customer agreed to pay lives in the contract, so make sure it covers:
- What the fixed charge includes. For fire alarms, BS 5839-1:2025 puts servicing at roughly six-month intervals, so two visits a year inside the fixed price and everything reactive outside it gives a clear line.
- Rates for call-outs, out-of-hours work, labour and parts.
- That fixed and variable charges will be collected by direct debit, on which day, and that the invoice acts as the advance notice.
- A pre-approval limit, for example reactive work under £250 billed automatically and anything above it signed off first. Customers rarely dispute amounts they've already agreed to in principle.
Neither BAFE nor NSI sets rules on payment method, so the direct debit terms are yours to write. If you hold NACOSS Gold, your terms already state the price and annual charges; add the collection day and notice period.
On the invoice, put a standard line such as "This amount will be collected by Direct Debit on or after 20 October 2026", and give every variable line its site, job number and date. Close off job sheets on a fixed day, say the last working day of the month, so the invoice date and notice date stay steady.
If you'd rather not build each month's collection by hand, Adfin collects varying amounts under one mandate from the invoices you already raise, whether you import them from Xero or QuickBooks or upload them, and matches each payment back to its invoice as it settles.
When a collection fails or a line is queried
A collection can fail because funds are short on the day, the account has closed or the customer has cancelled the mandate through their bank, which they can do at any time. A cancelled mandate doesn't end the contract. The charges are still owed, and you can collect them another way until a new mandate is in place.
For a query, have the job sheet, engineer's notes and any photos linked to the line so you can answer the same day. If one line is disputed before the collection date, reissue the invoice without it and bill the disputed amount once it's settled, so the rest of the month's money still arrives on time.
Failed collections and unsigned mandates create the most repetitive chasing in fixed-plus-variable billing. Adfin retries failed direct debits for free and can offer the customer a card payment if the debit still doesn't go through, and its AI agents chase unsigned mandates and unpaid balances over email, SMS or WhatsApp on the schedule you set, with you approving what goes out.
Common questions
Do customers need to sign a new mandate every time the amount changes? No. One mandate covers varying amounts on regular or irregular dates, as long as you give advance notice of each new amount and date.
How much notice do I need to give before a variable collection? The Bacs default is 10 working days, plus postal time if you post the notice, unless a shorter period was agreed with the customer when the direct debit was set up.
Can the invoice itself count as the advance notice? Yes, if it states the amount and the date you'll collect it.
Should the fixed charge and call-outs be collected together or separately? Either works under one mandate. One combined monthly collection is simpler for most customers; separate collections suit those who want to approve reactive work before it's debited.
Can I collect more than the amount I notified? No. If the amount goes up, send a new notice with the revised figure and allow the full notice period again.
What happens if the customer cancels the mandate halfway through the contract? The contract still stands and the charges are still owed. Invoice as normal and collect by card or bank transfer until a new mandate is signed.
Sources
- Bacs - Direct Debit: an introduction to the service (accurate as of September 2026) https://www.bacs.co.uk/media/pu4bmlzs/dd_introduction.pdf
- Bacs - FAQs (accurate as of September 2026) https://www.bacs.co.uk/resources/faqs/
- Pay.UK (Direct Debit) - Direct Debit Guarantee (accurate as of September 2026) https://www.directdebit.co.uk/direct-debit-guarantee/
- Fire Industry Association - Guide to the changes in BS 5839-1:2025 (accurate as of September 2026) https://www.fia.uk.com/static/77c28e9f-486e-4b96-a82e4bb37f4f3882/Guide-to-the-changes-in-BS-5839-1-2025.pdf
- National Security Inspectorate - NACOSS Gold approval criteria SF 002 Issue 9 (accurate as of September 2026) https://nsi.org.uk/wp-content/uploads/2012/06/SF-002.9-NACOSS-Gold-approval-criteria-Oct-2024.pdf
- Adfin - Direct debit payments (accurate as of September 2026) https://www.adfin.com/direct-debit-payments
