According to this:
http://www.adviceguide.org.uk/index/...No_P45_or_P45U
you can't get a duplicate P45.
However, you should get a P46 from your employer, which should sort your tax levels out.
HTH
EDIT: Just read your post properly
. The P46 will effectively replace the P45, which will get passed to the Inland Revenue to sort your tax out