Is there a way to check that my Margin is correct on the Invoices and jobs as I am processing them?
Yes. You need to make sure the Hide Costs in Product Enquiry option is unticked from under the Admin \ Defaults \ Manage Product Defaults menu
To see the Profit on jobs go to Admin \ Defaults \ Manage Job Defaults - Cost Entry, deselect Automatically Authorise Costs.
Once that is done, when you enter an invoice or job you will see the Show Cost/Profit button. In Version 8.300 onwards the lines show the individual cost and markup. The figures at the bottom of the screen show the total of the invoice.
Select this to see the Profit / Margin for the invoice.
It will also show at the bottom of the Manage Invoices screen.