I think the point, bauhausfan, is that Ofsted are there to make judgements about all sorts of things - one of which is the quality of the teaching. They can only really do this when children are being taught - and often in the last week of the Christmas term lessons are minimal, because there are a lot of exciting activities planned instead and because, due to said activities, children aren't really in a frame of mind for formal learning.
I personally think there is a lot to be said for making the last week of the Christmas term as short as possible for the children - Monday Christmas themed learning, Tuesday Christmas Concerts, Wednesday whole school assembly and play based activities, Thursday and Friday as INSET. It would save having a whole 5 days swallowed by the insanity...