From CIMT, see their Further Mathematics Chapters, part of the A Level series; note the 6 sections, Pure Mathematics, Statistics, Mechanics, Further Pure Mathematics, Further Statistics and Discrete Mathematics.
If you have not come across the HELM Project before, the project was designed to support the mathematical education of engineering students and includes an extensive collection of notes which include very clear worked examples. For easy access to these resources, the HELM Project Workbooks are hosted by Loughborough University’s Mathematics Learning Support Centre. Alternatively, the complete set is hosted by the Open University. To access the Open University resources you will need to create an account (easy and free), this will also give you access to the numerous free online courses.
Check these very clear notes with exercises from Plymouth University. This project includes a library of materials, some of these are useful for A Level Mathematics and Further Mathematics. Clear instructions are provided for using the documents.
A further source of notes and examples for a small number of topics for Further Maths comes from Mathisfun, for example I have often used the clear explanation from this site for the inverse of a 3×3 matrix. Scroll to the bottom of the page and there are 10 examples to try – all with fully worked solutions.
There is a clear index by year and subject and if we look at 10 upwards (Year 10 to University) we can see what is available. Under Number for example, we have Complex Numbers and under Algebra we have Partial Fractions.