Dynamics NAV 2016 and Web Services

Good Day,
I am having an issue whereby I create a sales invoice header and then the sales lines but NAV 2016 has the non-editable VAT fields in the fast tabs now and C# brings back an error that the field cannot be updated because it is non-editable.
This is a calculated field in NAV and I am not passing anything to those fields - just the Item Type, Item No and Qty to the lines.
If I make the fields editable then there is no issue but this is not what I want else users can change the figures in those fields.
Please can you steer me in the right direction with this.

Thanks very much

Thomas