In such situations always gather evidence and hire a lawyer. HR are there to protect the interest of the company, you need someone that you employ to protect your interest.
The union least has lawyers experienced in this kind of trouble, and they are cheap/free to use for members.
This is still good advice though - unions will probably help you even as a non-member.
Ref: https://www.worker26.com/ & https://twitter.com/worker291
Disclaimer: No relation to N26 or the union in any way, just another Berliner supporting their cause
Most unions I know of operate on a country-level.
They have an advice line, but don't generally help non-members.
https://prospect.org.uk/article/getting-help/
UTAW looks like a new British union for tech workers, set up by Google and Microsoft employees.
The purpose of the lawyer is so that you don't have to invest your brain in that issue. Your lawyer can deal with the intricacies of court procedures and employment law while you move on with your life.
There may also be a moral duty of injured employees to sue their employer because corporate behavior won't change if they don't. Letting the company get away with it is like letting a rapist or child molester get away with it. There will almost certainly be future victims so if someone can put a stop to it, they should.