With thanks to Mr Rooney for spotting this:
http://news.bbc.co.uk/1/hi/uk_politics/5056200.stm So, either the tax works and people don't drive... meaning there's a tax black hole, which will be made worse by the knock effect on jobs.
Or, they get the revenue and the environment still gets destroyed.
Still, at least it does show some sort of commitment to the environment - they re-used a fag packet to write the policy on!