Thew following day, post to wip worked, all on it's own. We then needed to post some more transactions and the same thing happened.
It turns out that whilst receiving the error message, Clarity was actually clearing down the transactions still. We continued getting the error message through out the day on another couple of occasions where there were more transactions to post to wip, even though it was working.
There were no locks in the prlock table, and we hadn't already begun posting to wip
Such fun.