Making the PO Number mandatory on checkout ensures you capture the Customers payment method.
Visit Admin -> Configuration -> Payment Methods
Locate Purchase Order in the list and click configure
Make the PO Number compulsory by checking the box.
If your storefront has hidden pricing or £0.00 cost items, it may be beneficial to also check the Enforce Payment Workflow box. This forces the Payment option screen regardless of basket cost on checkout.
Save any changes