When you pay a chartered accountant to file your vat return, crop tax and annual account, you effectively transferred risk of any potential penalty from authorities due to misinformation and/or wrong calculation. Although this does not entirely remove your responsibility and any legal actions against you, it does mitigate it to some extent. You may be confident with whatever your filed to HMRC and company house are legally compliant simply due to the fact that you could not tell whether your calculations and presentation are compliant.
If I were you, I would pay the professionals because my time worth more than what I pay them as well as by paying them, I transfer the risk of penalty imposed by authorities.