How do I add a custom field after VAT field

I need to add a custom field on a sales invoice which needs to appear after VAT, this is to show a CIS deduction payable by the client, how do I do this?
read the guide on line item custom fields.

positioning of fields is currently possible only with the help of custom themes.

Thank you but I think that I need to create a new tax code rather than a custom field. It is not a case of simply selecting ‘Deduct withholding tax’ as this deducts an incorrect figure.

Sub total £100
VAT 20% £ 20
CIS - 20% -£ 20

Total £100

Previously I have been showing this as a line item but this creates an incorrect sub-total.

Looking at some of the guides it seems that it is possible to show additional tax lines but when I have tried to create a new tax code it is not appearing correctly.

@Asthill, you need to explain further. What is CIS? Who is it paid to and for what? How is it supposed to be calculated?