I have seen comment suggesting an average of 62% turnout, however in one particular scheme where s/h were actively contacted to vote turnout was 88%. So, think you're pretty much on the money.
Yes, 17.52% is 25% of a 70% turnout. Tiga's stake in such a case would be able to block - but not if turnout was, say, 80% and all other s/hs voted FOR the Scheme.
Don't think Iress have any shares. If they did hold any, as you suggest they would be excluded from voting and would reduce the pool of shares able to vote.
OVH Price at posting:
38.0¢ Sentiment: Buy Disclosure: Held