When I had severe headaches earlier in my pregnancy (and my previous pregnancies), 3 different doctor's told me I could take Panadine (which has codeine) but not for a prolonged period of time (I think for only 24 hours with the recommended dose). So unless all 3 doctor's are wrong (the most recent is my Obstetrician) I believe codeine is ok. I would definitely check with your doctor or midwife though. Panadeine Forte has a larger amount of codeine than Panadeine.
Btw, Codeine is classed as a Category A drug so safe during pregnancy. The only thing I have read is not to take it during labour if your baby is pre-term. Again, check with your doctor and good luck!
[Edited on 19/09/2008]