The idea of the Flat Rate Scheme for VAT is supposed to be to make record-keeping easier for businesses. Record-keeping for most professionally-run small businesses isn't that hard anyway, so in reality the big winners from the FRS are those businesses who manage to park themselves in a business category that results in their paying over a smaller amount of the VAT they've collected from customers than HMRC would like.
This is very common, because the list of FRS categories is pretty poorly drawn up, and is full of gaps and ambiguity. HMRC's answer over the years to this has not been to redraw the list of categories so that it's better, but to put some guidance behind it saying what categories HMRC believe should be used when the nature of a business doesn't fit the list (which in our experience is probably at least 25% of the time). The thing is, the guidance has absolutely no legal basis at all. HMRC haven't accepted this, and have tried to enforce back-VAT and penalties against businesses who went on the basis of the list, not the guidance - the classic example being a mechanical engineer, who chose the "other" category since mechanical engineers weren't listed. HMRC insisted he should have used the "civil engineer" category. They lost the case, because he explicitly is not a civil engineer, so "other" was accepted by the tribunal as being far more appropriate.
HMRC have now realised that they're losing that battle, and have now accepted that they can't go back and collect more VAT as long as your choice of category is reasonable. What they haven't said is whether some day they might just amend the list and make it functional!