Woodmouse, The standard of language teaching in primary schools is inconsistent at best, appalling and inadequate at worst. Primary schools drop languages at the drop of a hat for all sorts of reasons, such as sports day or cramming for SATs, because very few take languages seriously. Most of them don't have teachers capable of teaching any language to a reasonable level.
Secondary schools with a number of different feeder primaries can't rely on their new pupils' having reached a reasonable standard. In some cases, they haven't even learnt the same language. They have to start from scratch, because they really don't have much choice.