If you are a subcontractor under the Construction Industry Scheme, your contractor takes twenty percent off your labour before paying you and hands it to HMRC against your eventual bill. Thirty percent if you are not registered.
That money is not gone. It is tax you have already paid, and it comes off what you owe at the end of the year. Plenty of subcontractors end up owed a refund because of it.
Where it goes wrong
Not in the arithmetic. In the record keeping.
The deduction appears on a payment and deduction statement from the contractor, which arrives monthly, on paper or as an email attachment, and goes in the van. Come January you are trying to total up twenty percent of labour across eleven months and four contractors from statements you may or may not still have, and the number you put on the return is the number you can evidence rather than the number you actually paid.
The gap between those two is money.
What Books does
Records the deduction against the job when the payment comes in. Which contractor, what the labour figure was, what was taken, what you actually received. It sits with the income rather than in a separate pile, so the total is a total rather than a reconstruction.
When you need the figure, it is there. When HMRC or an accountant asks what it is made of, so is that.
What it is not
It is not your CIS return and it does not file anything with HMRC. It is the record that makes filing straightforward, whether you do that yourself or hand it to somebody.
Books is fifteen pounds a month and now works for any tradesperson in Great Britain, whether or not you ever take a job through us.
The honest caveat
If your contractors are diligent and you keep every statement in a folder, a spreadsheet does this. The reason to use something else is that the deduction gets recorded at the moment the money moves, when you have the statement in your hand, rather than eleven months later when you do not.