Even more difficult now HMRC got rid of all the local experienced tax inspectors working in local town centre HMRC offices (which have been closed down). The kind of people who lived and worked in their local area, so could use their local knowledge based on what they observed from going about their normal lives, and what they gleaned about other businesses by going to random businesses to "check their books".
Back in the 80s when I first started in accountancy, we'd have regular visits by tax inspectors coming into our office, almost on a daily basis to check the "books" of clients we acted for, usually for VAT returns, but also for payroll. They'd make notes and take copies of B2B invoices etc in our clients' records and then cross check them against the other businesses' records at a future inspection, just to make sure that the same invoice and same payment was recorded correctly in both!
Or the local inspector who would get one of his assistants to log down details of deaths as per the local newspapers, then check undertaker records to make sure they tallied, and then checked the care home's records to make sure they tallied too! He discovered several local care homes who were only recording income from, say, 8 residents, whereas the home had 10 rooms!
Another one would make notes of local boarding houses/guest houses showing "no vacancies" and then many months later check their books to check that they were showing a full occupancy in terms of booking income!
Quite devious, some of them! Especially the one who'd go to random pubs in evenings whilst he was off the clock, as he liked socialising, and would make notes of the amount of coins being put into the juke box, pool table, vending machines, etc., and again, months later go to check the books to make sure the takings tallied!
You just can't do that sort of thing based in a call centre 200 miles away! Hence why the "small business" tax evasion is growing fast. They know the chances of being caught out are pretty close to zero.