A lot depends on what you're good at and what you're trained for. There's no reason to suppose a train driver will be a good electrician, or a plumber will be a good book-keeper. Lots of small builders go bust (some of them repeatedly) and some are very bad at the business side.
I used to do my own accounts, meticulously, but still had a wrangle with the tax office once because they thought (wrongly) I had claimed too much relief on pension contributions. An accountant who has other clients in the same line of business as you, should know all the ins and outs, and be able to give useful advice. I'd say avoid one who tries to get too clever, or tries it on. If HMRC think he's sailing close to the wind, they'll get out their big magnifying glass to examine all his clients. Even if you are in the clear, it wastes your time.
I believe they use a model of what they think a typical person in your line should earn, what the materials cost and expenses should be as a proportion of revenue, and so forth, and if you have anything unusual about your patterns, they will want to see why.