On Fri, 15 Sep 2017 08:19:08 +0000 (UTC), d wrote:

Looks like it could be an improvised device, or some builders chemicals that
overheated. Hopefully the latter but seems unlikely to me.

http://www.bbc.co.uk/news/uk-41278545

It's now in court:

"the device was made from the volatile chemical explosive TATP and
contained 2.2kg of sockets, screws, bolts, nails, knives and
screwdrivers"

http://www.bbc.co.uk/news/uk-43318098