Thanks, I looked at that, but (although I did say basic!) it is a bit simple. I think I might be getting somewhere by just deleting a whole bunch of stuff out of the template...now to figure out how to set a tax rate! Thanks again for the suggestion, you never know, I might end up reverting to that! :)
You may find that adding to the above demo is less effort that removing stuff fromt the template.
Tax rates can be stored in either a global field or in a related table that always matches to any record in your invoices table by using the X operator.
You can then add a tax calculation field to Invoices that multiples the total of the items in the portal (Sum ( LineItems::ItemCost ) ) by the tax rate.