I'd raise this with yoyr manager about policies you should have some sort of company hand book but you will be on probation first usually 16 weeks before you are classed as a permanent full time employee.

During this period you will not be entitled to book any holiday but may take holiday pre booked before joining (for example holiday you booked with your previous employer last year).

you shopuld have a appraisal after your probation where they can discuss the future of your employment until then they can dimiss you when ever they want to unfortunately as this period they can hide behind company employment policies, if your really concerned it's best you join the workers union.