Sorry to hear what you have said.
Of course there is a working time directive. All employees are subject to it, even if they have signed a document as an opt-out.
Of course if you are self - employed then you have to set up your own working hours in your contract.
Even if you are part - time then you are protected, for hours, pay and holiday pay.
Please check the DTi website and if need be take some legal advice.
If you feel that the flying is becoming dangerous then you need to act.
Remember you are training a student, and if you are not getting the training right you should walk away.
Since I trained as an instructor I have realised that the school I learnt at did not give me the training I was paying for, no long briefings, little or no pre-flight briefings, just get in the aircraft and fly. Not the best way of training.
MAke sure you have a contract - if he complains then I would suggest that there is something else going on and you should be wary and maybe even look elsewhere.