The evidence we have when we look at these things after each election is that, on average, for those who are in the international register, about 8% of ballots are returned late and so cannot be counted due to delays. For those who are not on the register, that number goes up to 15%, and that does not include the potential impact of the additional requirements that are built in here. I expect that number to remain and maybe grow a bit.
On May 7th, 2015. See this statement in context.