Staring new Job but no P45?

Your employer will give you a P46 form to complete. They will then send this off to the Inland Revenue to inform them you are now working for them so that the correct tax code can be issued. In the meantime you will be on a standard tax code (747L) but on a 'week 1, month1' basis which means it doesn't take your period of unemployment so far this tax year into account. However, this will be sorted out when your new tax code arrives so it will be evened out.