http://www.bbc.co.uk/news/uk-politics-16854187

And the BBC have just revealed that it was Danny Alexander who signed off this arrangement - something which he refused to admit despite being repeated asked in the HoC todays. Chris Willets also agreed to the arrangement. Rumours now that Labour ministers agreed to similar arrangement during their administration.

How can all three parties in force austerity whilst, at the same time, giving tax relief of the most direct nature to the well off?

Because in the days of yore (well, think it was 19th Century), the ruling elite didn't flinch from imposing austerity on the masses. It's what they do.