They will ask for details of one year, probably the one for your last tax return. They will ask for all relevant paper work records and bank statements and paying in stubs.
When you take them in they will arrange an intial meeting where you explain the basics of your business and how you typically handle cash and payments.
There will be further meetings, and they can request other information to corroborate what you've told them.These meetings go one for hours even if you have nothing to hide so it's easy to see how both preperation and attendance fees will probably be over £2,000.
They do three tests, cash, margins, and lifestyle.You can fail all three and still be okay, but most people i know end up paying something to bring an early conclusion to a stressfull process.