The Royal Society has put their commissioned report on ethnicity in STEM online. You can find the full report here

The aim of the report was to look into why “talented black people are not finding science careers in UK academia”. The key findings of the report can be found in more detail in the above report but, in brief:
1) Black, Asian and multi-ethnic candidates are less likely to apply for Royal Society early career fellowships (even when considering the small numbers eligible to apply).
2) These groups are under represented in the eligible pool and applications to Royal Society schemes.
3) There is “little to no” representation of Black postdoctoral researchers.
They highlight some possible steps forward including mentorship and workshops. The full reports include thorough data and analysis.

Royal Society report on Ethnicity in STEM