The NTP server that ultrablockparty.com was pointing to is 7 minutes off of pool.ntp.org. I've reset the clock manually and have stopped using the NTP server that the host has provided as it is clearly misconfigured. We'll just have to tweak it manually until the provider can be persuaded to make a working NTP server available that is on the same time as the rest of the world.
This problem will have much less of an effect once we leave the exponential retarget that is in operation now and implement the new difficulty system that averages out over more blocks with the new wallet.