So, ios files deleted and 'applications' now showing approx 6GB's.
Using the link you provided, the iPhone backup is gone (as expected). Everything back to normal, except I've no back up of the iPhone now ;-) There seems to be some accounting error as you said, but as to why, it would appear to be something software related, rather than something I've managed to do. Perhaps it's a bug that arrived in one of the last few updates and Apple haven't got round to fixing it.