The tax year ends in December, so the best you will do is eight weeks addtional allowances. Revenue may entertain a reclaim when you get P45. I think you use a P50 to get tax back.
You dont have to wait until the end of the year to get tax back. You could register with taxback.com and they'll tell you what you are due. They charge 12% but it may save hassle
Fill out this form, (fillable on screen) and print it out, send it to your local tax office along with part 2 & 3 of your P45 from your employer. Fill in bank details if you wish to have your refund lodged on your behalf while your abroad.