Chris Giles from the FT was the first to notice;
Really @statisticsONS can be annoying. Still got April's data set on its website. Come on ONS
— Chris Giles (@ChrisGiles_) June 27, 2013
Given all the issues the website has had over the last couple of years you would have thought we would be prepared for anything but this was a new problem and all the standard remeidial actions the development and support teams undertook were unsuccessful which is why there was such a delay.
The problem, once identified, was deceptively simple. Both the datasets that failed had character encoding issues due to unrecognised characters that essentially choked the automated process that creates and publishes the datasets.Now this has been identified we will be able to mitigate against it in the future but for now we can only apologise.