Ben gives ace advice.

You admit that they have some grounds, but not all grounds - so you are being reasonable.

Just chucking your job now seems a little unwise - plan your exit. Get another job, or something temporary if you can.

Get a copy of your employer's disciplinary, grievance, capability procedures etc - and make sure you understand them.

Most of all - don't give them any grounds for complaint. (I was recently talking to somebody whom was hacked off because he kept getting pulled up for being late - simple solution: don't be late.)