Chris - "the success relates to the starts made under Gordon Brown's premiership"
Only someone with a very warped sense of humour could regard anything that Gordon Brown did as a "success". The number of housing starts in the period April 2010 to March 2011 was less than the number of completions over the same period, indicating that the programme was already in a decline at the end of Gordon's "reign".
The reason the housing starts figures are so "abismal" as you put it is explained in the tables. The National Affordable Homes Programme, which was a specific housing funding project, came to an end in March 2011.
It was replaced with a new programme under the coalition, with the new Affordable Rent product (which had not been part of previous housing provision) included. Housing providers obviously took some time to familiarise themselves with the financial details of the new scheme before committing to new construction projects.