So it isn't a creaming off of the best, and it isn't based upon selection at any particular age.

Except that it is just that.  A school has to take children from more or less a given age, in order to prepare them for the all-important exams (the results of which, after all, are the means by which the pupils are able to show how talented or gifted they are to future employers/financial backers).  Otherwise, we just let the cream rise of their own accord, as per folk like Richard Branson and Alan Sugar.

As regard to the rising pass marks in exams, yes there are other factors which affect the figures, but you cant have 24 years of rising pass marks without the education system having got better at its job.

If the system was the same now as it was 30 or 40 years ago, this would be a valid comment, Martin: but it isn't.  Not only is the exam system completely different, but the social and technological context in which the education takes place is completely different.  Remember that technology has developed further in the last 50 or 60 years than it during the previous 5 to 600 years.  it is now the norm for pupils to take calculators into exams, and to have a selection of formulae printed on the question papers for them to choose from - both things that didn't happen when I did Maths or Science exams at school.

Late-20th and early-21st century education is a completely different animal to mid-20th century education - even if it is organised by largely the same legislation as the earlier animal.

...and that you would find much more evidence to support the idea that it has improved over the years.

Even the teaching unions and the Labour Party have ditched this argument, Martin.  Even they have acknowledged that 'improvement' is a meaningless word in this context.  Educationalists now talk about 'different' when discussing the issue.