At 10:53am UK local time, we saw our physical link to Level3 (AS3356) drop for 5 minutes.
This caused our routers to immediately terminate the BGP routing session with Level3 and start to reconverge via our alternative paths leading to 2-3 minutes of instability whilst routes updated.
The link to Level3 re-established at 10:58am UK local time, and BGP reconnected.
We have monitored the connection since the event with no noticable issues.
We have contacted Level3 to request further information.
Level3 have confirmed that the issue was due to a loss of power to their routing equipment in Telecity Manchester.