MATHS, in common with all other areas of the curriculum, can not be taught properly by those who possess only rudimentary knowledge of the subject.
The only way to improve standards is to make the profession an attractive proposition for potential applicants who are well-qualified, energetic, and enthusiastic. Then, and only then, will we see a really significant improvement in standards.
Whether standards have actually risen over recent years must depend upon whether you believe that appropriate comparisons have been calculated correctly.
Discussion over issues such as setting, streamng and mixed-ability is largely futile. Every set contains a mixture of abilities, and some sets are more mixed than some so-called mixed-ability groups. However, enthusiastic, well-prepared teachers with a thorough understanding of their subject can probably make any system work. It's always all down to the quality of the teacher.
Of course, there is little prospect of this or any other government providing the funds needed to improve matters; and so the number of poorly-qualified maths teachers is bound to increase.
44 Chester Way