Thank you very much. I'd thought I'd installed the April 11 and April 15 patches, but just to be sure downloaded and installed them again. Unfortunately the behavior is exactly the same.

Before I raise this in a support ticket, I wonder whether anyone would be willing to help me see if this can be replicated. I am happy to send the BIAB file if I can figure out how (I don't see a facility for attaching things from your harddrive).