Q. I've found another job - by when do I have to hand in my notice?

According to the Burgundy Book….

To leave by the end of the Autumn Term (31/12) you must hand in your notice by 31st October

To leave by the end of the Spring Term (at Easter) you must hand in your notice by 28th February

To leave by the end of the Summer Term (31/08) you must hand in your notice by 31st May