Yes you are right to be concerned, this could be quite serious.
You could start off by contacting the senior legal officer in your principal authority (usually the district council) who has a key responsibility for standards and will also have extensive experience of legal issues in local government. I would also find out who is your council's auditor and pass the information you have to the auditor, and ask them if they think an immediate investigation is appropriate.
These people may make further suggestions for action or other people to contact, but they are a good starting point.