Under Course Materials you will find an extensive collection of helpful resources such as the Exemplification of the New Sample Assessment Materials and the Topic Tests, note the Show More button which will take you to more resources including for example posters of formulae that students need to know.
Edexcel Teacher Support, includes many Practice Papers.
Edexcel Content Support includes Teaching Guidance, sample questions for new topics and Problem Solving questions. All of these documents are freely available to download with no login required. Note the GCSE 9-1 Mathematics Content Guidance which provides commentary from the senior examiner team for the new GCSE (9–1) in Mathematics content statements. Question references from the accredited Sample Assessment Materials and Specimen Papers (Sets 1 and 2) are provided to exemplify this content. The booklet of Sample Questions for new topics is very helpful
Also under Content Support you will find very helpful resources for teaching new content. For new content, information, examples and exercises (with answers) are given. (See this post for more on Iterative Techniques)
On Diagnostic Questions Edexcel GCSE Multiple Choice covering the specification are available – an outstanding resource.
Signing up to Edexcel’s Mathematics Emporium is highly recommended, note that it is a free website intended for the use of teachers of mathematics in secondary schools, regardless of what board you use. Register for an account and ensure you supply a correct centre e-mail address in your name for verification, your centre name and centre number.