Greek to English Art/Literary Names (personal, company)

Sorry if I appear to be insulting your intelligence with such a basic query. My problem has to do with transliteration, and I know virtually no Greek. From time to time I translate academic articles by Turkish Cypriot authors on recent Cypriot History into English. The transliteration of Greek names is a constant headache for me when I do this, because the accepted practice in Turkish is to provide the closest possible phonetic rendering, whereas the accepted system for transliterating into English is much more faithful to the Greek letters involved (for example, a place name transliterated into Turkish beginning ‘Ayos’ becomes ‘Agios’ when written in English). This means that I often have to trace names back to the original Greek spellings before I can transliterate them into English.

Now, a constant source of puzzlement for me in this endeavour is the way former Cyprus President Glakos Clerides’ (and let’s not get into a debate over whether this should be ‘Klerides’ in accordance with the ELOT standard – this is Cyprus!) father’s name should be spelt. I encounter both ‘Yiannis Clerides’ (oddly never ‘Giannis’) and ‘Ioannis Clerides’ – in fact, both spellings are used in the same Wikepedia article on Glafkos Clerides: http://en.wikipedia.org/wiki/Glafkos_Klerides . What I find strange is that these are transliterations of two different names in Greek, i.e. ‘Γιαννης’ v. ‘Ιωαννης’. Logically, you would think that only one of these could be his correct name. I have always opted for ‘Ioannis’ because this seems to be used in the most respected English language sources on the history of Cyprus. One possibility that occurs to me is that Γιαννης may be a familiar form of Ιωαννης, and that perhaps it is correct to use both in the way that somebody called ‘Charles Brown’ might also, informally, be referred to as ‘Charlie Brown’. If this is so, would it then be correct to use both versions as a stylistic variation in the same text? I would greatly appreciate any input you could give me on this point.