Last night the local news bulletin on the BBC had a segment on the poor rate of diagnosis of dementia in the south west. A striking feature of the report and the various speakers involved in commenting on the situation was the variability in the pronunciation of diagnose. Instances of the word were coming so thick and fast that I really did not have time to do a count, but my impression was it was evenly split between ˈdaɪəɡnəʊz and ˌdaɪəɡˈnəʊz.
LPD has a pronunciation preference result for this word (1993), but only for US English, at least in the only edition I can lay my hands on at the moment. The poll came out approximately 60/40 in favour of final primary stress for the word.