The IPKat's respected friend Gwilym Roberts has written to tell him:
"If you phone the Hague EPO on 0031 703 404500 you get a message saying "Welcome to the European Paytent Office". Does that make it the official pronunciation?"Never wishing to presume to give a ruling on this sensitive matter, and being quite unable to find the relevant provision in the European Patent Convention, its Rules, travaux and its guidelines which deals with how the word should be pronounced in English, the Kat has organised a poll, which you can find in the top left hand of this blog's front page, in which readers are invited to offer their preference.
Pronunciation of 'patent' here, here and here