I struggle to understand how part of your holding can be sold without your knowledge. Sometimes reductions can take place due to corporate actions but 75 is an odd amount to lose from 500. Which company are the shares in?
Sounds like a job for the
Financial Ombudsman Service. You'll probably have to raise it with the compliance departments of the registrar and the broker concerned first. As soon as they try to fob you off take it to the FOS.