ANSWER:

No need to start again as saying bismillah is not mandatory before making wudu, your wudu is valid even without saying it. 

if you forget to say bismillah in the begninig of wudu and remember it in the middle of wudu(for example in the part whene you wash your arms ) can you say it and continoue the wudu(wahd the face and what come after it )or you have to say it and do the wudu again (wahsing the mouth and nose) from the beginig