firstly her P45 will be dated in the year 2008 so you can't use that for tax credits or cut offs. Once a tax year has passed, its gone. What is important is when she is going to be paid. In this case, payment in January so you are into the new tax year.
Secondly, best thing is the ring the tax office and explain you have new employee, give her PPS number and they should immediately put her on to your company's books on a week 1 basis and give you the relevant cut off and tax credit figures.
I did similar last week for 3 new employees and got their tax credits/cut offs over the phone no problem.