longfingernails, that's a slightly weird document IMO, especially if it's being presented as evidence of Labour bias in the system.
It presents 5 aspects of the system that produce electoral bias, one of which is, er, the way people vote (tactical voting) which is hardly a structural bias, more a choice.
It states that UK population trends mean that boundary commission changes will ALWAYS mean a shift towards the Conservatives, due to the drop in the urban population, so that a delay in those changes means an inbuilt bias toward Labour. This may be true, but it's hardly an eternal truth - try page 14 of this certainly up until the end of the last century the proportion of the population living in urban centres was rising. This would explain why I remember discussion of the electoral bias towards Conservatives in the 1980s caused by delays in Boundary Commission changes...
It also states that the overrepresentation of Scotland has been eliminated (though not the West Lothian question of course), which should have improved things, even though the overrepresentation of Wales continues - this is not new, and at least things appear to be moving in a less biased direction.
Differential turnout - am I misreading this? It seems to suggest that if this element had been eliminated in 2005, Labour would actually have won more seats, i.e. this factor appears to be a bias against Labour. But I'm willing to believe I am misreading it.
Support distribution - they say themselves that this factor does not appear to bias very heavily to either side.
Yes I'll believe that the current system is biased towards Labour, I have read it in many places. But I don't find that particular analysis particularly convincing.