Sorry to hear that your boss has become a bully. One would have to wonder why after three years has the boss decided to not listen to anything you say in terms of making the department run as well as I am sure it has. Some suggestions I can offer:
Schedule a one-on-one meeting with your boss. Let your boss know before the meeting why you are interested in this get together. Point out that you have concerns that need to be addressed. Giving your boss a fair warning before going in may make him understand that something needed to be addressed.
Not sure how large your organization is but checking in with your human resources and/or personal office would be a good step as they are there to assist when after meeting with your boss may not resolve the issue with his hostility.
Understanding what makes your boss tick can help in managing his outbursts. Get to know his likes and dislikes inside and out so that you can avoid future criticisms.
I've raised my concerns by emails and a couple of meetings with his boss. I spoke a quite few times with the legal Officer at work. The feeling is that they know his attitude. The only thing is that his superior gives the impression he is on his side for obvious reasons. As you said he is a bully and everyone at work knows him.
The company employs about 150 max globally.
I am thinking if there is a way to raise a complaint to an Employment Tribunal while I am still at work. They then WILL listen to my grievance.
Sounds like you have acted professionally which is what should be expected in your situation. Continue to do so and try to see what may "push his buttons" and try to work around that when the situation comes up.
If you have an opportunity to talk to a tribunal of some sort, then by all means do so. Before doing so, references everything beforehand and keep a journal with the times he has acted as a bully and not treating yourself and other co-workers in a professional way. Having information detailed before going into the meeting will give you the edge and help your cause when presenting your facts in the meeting.
On what ground the complaint to an ET, if any, can be lodged ?
That I am not familiar with so went ahead and linked to information you can review to see if a case can be made in your situation:
Review this site before entering any grievance with the boss and your company. Continue though with being professional in the office and see what may make the boss go off at any moment and try to not cause this outburst to occur often. Difficult I understand but seeing what makes the boss act as he does may help in having a better work environment. Do not do this alone as help from your co-workers is key. Keep detailed notes and use the tribunal as a last resort.