Short summary of the article. The outage was caused by the introduction of new requirements by LSE for end-of-day price reporting. The data vendors were the ones who had the responsibility to adhere to these new rules, by making changes to their systems. Unfortunately, the "big bang" approach to the switch-over proved fatal.

From the article:

“I am astonished they did not run the systems in parallel,” said a source at one of the major data vendors. “At least for a few weeks or months as necessary.”

The LSE argues that it gave plenty of proper testing and preparation time, and that the vendors should have been ready. It has been working with them to resolve the issues.