Hi Chris,
Have you looked at MotorDesk?
We provide a DMS with sales and invoicing tools. This is of course primarily intended for 'standard retail' but it's also ideal for individual users to manage stock, create invoices etc...
MotorDesk provides live synchronisation with Xero, Sage and QuickBooks, so whenever you create or edit an invoice it will be updated in the connected accounting software automatically, within a few seconds. Through this integration we're also able to automate your marginal VAT calculations so you don't need to mess about with manual changes or journal entries.
Let me know if you have any questions or if you'd like to see a demo!
Neil