Wish you luck and I hope you have taken good because it looks to me as if you may find this difficult case to win, altough they might find the employer failed to respond propoerly to your grievance. Sadly, although no prospective future employer will admit it they won't be keen on taking on someone who has takena previous employer to a tribunal.
I'm puzzled about your being told you need to get permission in writing for overtime- had you been doing overtime without prior approval?
And are you part time- some employers only pay normal time not time and a half for overtime worked by part time staff.