Have you been in touch with WorldPay? Their support centre maintains everything is OK and all their systems are 'Normal', although we've been unable to process any orders since 1pm this afternoon, but we've been having problems since Saturday afternoon.
We've setup a temporary secure server to collect orders but we can process anything until WorldPay comes back online.
Luckily we're in the process of setting up our own merchant account and that should be ready in about a week but we need something now.
