Sage - Overwrite Vat

Hello.

Not sure if anyone has any bright ideas on this one.
Someone is trying to generate an invoice for me on their Sage. She is on Cash accounting.

Now I need an invoice for insurance claim. Policy Excess which is not vatable and Vat paid but with a zero cost.

So as follows.

"Policy Excess" Nett £250 - no Vat
and
"Vat paid by customer" Zero cost but £415.40 vat.

She can generate the first line for policy excess.
But the Vat paid by customer, in the Customer - Invoice - Edit Item line box - she cannot do 1 x Zero and then overwrite the Vat.
On mine I can overwrite the Vat.

Is this anything to do with Cash accounting do you think?
Any ideas would be appreciated.

Yas