When I finalise the payroll it posts the journals to the linked accounts but how do I get it to post each Employee by class and how do I assign a class to an employee?

Hello there, bob104.

Right now, assigning a payroll class to an employee is unavailable. You may wish to consider looking a third-party app that's capable of doing this. You can go to the  App store page and just change the flag to the UK to get the list of apps for your region. 

However, if you currently subscribe under QuickBooks Online Advanced Payroll, then you can assign classes to location. To know more about this, consider checking this article: Classes in QuickBooks Online Advanced Payroll. 

Get back to me if there's anything else I can do for you. I'm always here to help. Have a great day!