Hi Nick,
You'll have to use the simple start product, to get the functionality you are looking for. Effectively, the Self Employed product, is there to help you fill in a Self Assessment Tax Return, and provide other useful, albeit basic, business functions.
More advanced features such as multiple invoice styles, custom fields on invoices, i rarely see a purchase order field on invoices (titled as purchase order) - i see "Supplier Reference" moreoft, however you can utilize this using the custom fields function in our Online Packages.
Finally, if you do need to create a Self Assessment Tax return, but want the functionality that Simple Start, you can have an accountant set up your Chart of Account to match the Self Assessment tax return categories - we can't support you to do this.
