Hi Neil
It really depends when she would receive her first payment. I'm assuming for this reply that it's 2009. In this case, you're only concerned with 2009 (i.e. forget 2008 execpt for the need to give her a P60). So until she's registered as your employee and you receive a tax deduction card (TDC), she's on emergency tax and your payroll system will tell you how much to pay her.
When you get the TDC, you'll pay her using the credit and SRCOP given on the TDC. Her credits/SRCOP in Jan will be 1/12th of her annual credits. In Feb, she will have 2/12ths cumulatively etc regardless of when in the month you make payment.
I hope this makes sense. Payroll isn't exactly straight-forward and if you're not entirely sure what you're doing, I'd definitely suggest you take a course or outsource it.
I hope this helps....