Hi @Rory
You would need to enter this as 2 separate lines - one for the net plus VAT, and one minus the net with 0% VAT.
My colleague outlines this better, here: C79: Import VAT / Import Duty / Handling Fees - #2 by Joe. But you essentially add the net and VAT on, then take the net back off, leaving you with a VAT only invoice.
The difference here would be you’d have an additional line for the £45.00 customs charge.