Ask HN: Recurring payment profile, PayPal sandbox, not billing?

I've got a ticket logged with PayPal currently, and I've seen reference in 2011 to a similar issue, but does anyone use the CreateRecurringPaymentsProfile with Express Checkout in the Sandbox and see it not billing?

I've got a recurring profile setup (and can see it in both the seller and buyer accounts in the sandbox OK) but the "next billing date" is set to 2 days ago (when I created it, the same as the start date of the profile) but the billing hasn't gone through (ie I can't see the funds either gone from the buyer or into the seller accounts)

I'm not using IPN (relying on periodic polling of the PayPal API to check the status of the subscription) but if I can't mimic the entire subscription process and check the periodic billing is working, I can't guarantee that everything's doing what it should - anyone using PayPal in a similar way and have any advice?