OK downgraded to 0.9.9 and tried it with this version of the file and it is as far as I can tell just as bad. Once a few splits are put in, those files can be instantly accessed but double-clicking the large file remnant again produces a spinning blue circle. So it does seem to be objecting to the file size despite the fact that it has only had mostly minor text changes to the Word original, the whole thing has been regenerated from scratch and an extensive clean up performed which is identical to every previous version when all was OK. (Said cleanup removes a lot of the Word-generated rubbish style info, for example, and replaces it with a one line reference to a separate CSS.) The epub check is fine. As the problem goes away once the splitting is done will just have to put up with it.
