The tax year should start at the beginning of the year, January 1st, and all changes to tax rates, NI etc. take place at exactly the same time. No changes during the year.
Why does this idea matter?
I am a very small employer and do all my own 'paperwork' not only accounts, PAYE etc. but also annual records of environmental and animal legislation. The tax year start date of April 6th is historical, but a modern anomaly, when nearly everything else runs from Jan 1st to dec 31st.
Every year on April 6th I have to start a new wages record etc. HMRC send me a CD with all the changes on, which I load on the computer. The blow me a few weeks later HMRC send another CD with the budget changes (and associated letters etc.) to take effect from May 18th. What a complete waste of money and resources. If you are going to make changes at all make sure we only have to update once during each year.
I'm really surprised no-one ever mentions this as a quick way of saving the nation money. Yes, I know the budget is not usually until just before the new tax year starts. So change that sraight away.