That is completely false about it being marginal.
Even with accounting fees im still saving money and you have the option of getting the best price by shopping around if you so wish. You cant shop around for the best price as far as tax is concerned.
You do need audited accounts, but there is a lot you can do yourself to limit the costs. Many accounting packages theae says are very cost effective and simply need a check at the end of the period by your accounting firm before accounts submission. Not like it used to be having to pay for a bookkeeper anymore. Some companies offer a whole package electronicly now.
My accountant is charging approx £1300 a year and that includes payroll but they are a chartered firm Barnbrook Sinclair who i highly recommend with those fees a decent accountant will be well worth thier price.
As a sole trader you are unfortunately open to all sorts of abuse from HMRC. sole traders are an easy target, you might think by doing things yourself you are saving money but if HMRC decide to target you on an investigation it can get ugly as you have no legal representation or comeback and if they can get away with it they will argue that white is black, trust me i have been on the receiving end of this its not nice, and they dont care at all.