Employer Breach of Contract?

I have been with my company for 3 years on a Tier 2 Visa, recently renewed and my biometric pass received.
I believe my employer is breaching my contract as I have worked 85 Overtime (apart from working 9-5:30 Full Time) the last month and a half. There is no compensation, no days in lieu, no holiday. My time is demanded, not asked. Secondly, I have several sexual harassment complaints, however we do not have an HR department in our company. I have no benefits and no bonuses. My notice period is three months. I have to pay back my sponsorship fees once I move to another company. I feel completely trapped.

My questions therefore:
1. Is my boss breaking the Law and therefore does this mean it's a breach of my contract/
2. If so, can I leave without serving the full 3 month notice period?
3. What are the implications of leaving the company without serving 3 months?
4. What are my options with regards to finding another sponsor?

Any advise would be really great. Thanks

Your post is more to do with employment law than immigration.

It is not possible to answer your questions 1-3 without sight of your employment contract. A grievance can be raised, and an employment tribunal claim can be made, even if there is no HR department in the company. The procedure should be explained in the staff handbook or your employment contract. However, be aware that even though you have worked for the company for more than 2 years and therefore are eligible to make a claim for unfair dismissal, if you embark on this course the company may end your sponsorship.

If you want to pursue the rather serious matters alluded to in your post it would be advisable to consult an employment lawyer.

You can look for another sponsor but don't tell your present sponsor if and when you do and, following the assignment of a new CoS, don't resign before your application is approved.