Cart doesn't clear after purchase??

Hi,

We've set up MarketPress to use chained payments, and only chained payments. We can purchase products just fine, including heading off to PayPal, and coming back to the site again. Payments go through, and orders are generated, However there's a small niggle in that the cart still appears to be populated - ie if I go from the order confirmation screen to "Shopping Cart" - it still has the products in that I'd just bought.

From looking at the code - it looks like the call to set_cart_cookie(Array()); should clear this out - but it doesn't seem to be working - although everything else in that chunk of code seems to be running - so presumably it's being run.

Anyone got any ideas on next step to debug this - or seen it before?