Indeed it was once 2008, if you trawl through some of the newspaper articles from the original announcement. This is another seriously delayed major zoo project, but the reasons are probably complicated because it is part of a much larger development at Silvertown Quays. The projected opening date is 2010 or 2011 depending on whether you believe ZSL or the Silvertown Quays developer. There won't be an acclimatisation period beyond the necessary testing and commissioning of the aquarium systems and the initial cycling of the tanks. On an £80 million development (and there's another number that's creeping upwards), ZSL will risk losing a massive amount of revenue if they delay opening.
I think some of these recently announced projects at UK zoos are very ambitious. Often, the zoo will spend a relatively small chunk of money developing a masterplan and getting a planning application. But actually securing the finances for the capital cost of the project is nearly always the stumbling block. One reason American zoos are more successful at pushing through big developments is that there is a culture of big donations from rich individuals.
I really do wish the best for Chester's Superzoo but they are going to have a hard time raising £300m.