Bill wrote a good answer. To add to that , this is quite in involved task. It is something i am working on adding to convert-ixbrl.co.uk , adding the shareholder info as an additional piece of information along with company accounts json that the api returns (or what the web portal shows)
A- CS01 doesn’t always have share holder information. Sometimes the information is only found in the original company reg memorandum. This means checking the filing history of each company, all the way back to its creation to analyse the share holder changes. In additiona to that , sometimes companies submit ‘confirmation statement with shareholder information’ which contains this info.
B- For many companies , the memorandum is a PDF. Some of these pdfs require OCR making the task more complex
C-If the number of companies that you need is large, there is then the challenge of the api limits. For a few hundred or perhaps even a few thousand companies you might be able to work through the filing history within a reasonable time, without going over your call threshold.
D- You would need to put a process in place to keep this information up to date obviously, so that’s a whole data pipeline right there.
If you come up with a novel way to do this, please do share it here 