My dad is called Petre and we had always assumed that it was a Welsh variant. However I have to say that it is one of those names that is always pronounced wrongly. Many people have tried to sell things over the phone to Miss Petre Jones. Generally they pronounce it like the old Blue Peter dog: Petra.
Source: https://en.wiktionary.org/wiki/%E1%83%9E%E1%83%94%E1%83%A2%E1%83%A0%E1%83%94 (in English) [noted -ed]