PayPal Express Ninja Forms extension - how change currency?

I have a form set up through Ninja Forms and the PayPal Express extension. I'm currently testing the form and all works as expected (on submit the user is redirected to the PayPal payment portal, payment is processed successfully and it shows up in my PayPal sandbox account).

However, the amount the user has to pay when they reach the PalPal payment portal is in USD, but I would like it to be in GBP. Is there a way to change this? I have the primary currency set to GBP in my PalPal sandbox account, and I can't seem to find a way in the Ninja Forms settings to set the currency type? In the settings for my PalPal Express action I am getting the Total from a number field.