What are the alternatives?

Doing it yourself is not easy. It is difficult enough to master HMRC tax tables with several tax bands currently in force. To add further complication, most nannies’ wages are agreed as net (i.e. take home) pay, while all tax calculations are based upon gross pay, never mind trying to become an expert in employment law!
Not declaring employment to avoid tax is a criminal offence which can potentially result in heavy penalties and career–damaging publicity. It also affects future pension and benefits entitlements of your employee under the National Insurance system. HMRC no longer turns a blind eye to this issue.
Just using an accountant is usually much more expensive than using Nannytax. Add to that the cost of using a lawyer if the employment relationship turns sour and you could be looking at ten or more times the cost of an annual Nannytax subscription. That’s why even accountants and lawyers use Nannytax!