From the network status page:
BT Wholesale have announced Planned Engineering Work will be conducted on the BT Broadband Network on 4th March from 00:00hrs through to 06:00hrs. During this period there will be a 60 minute downtime to End Users. The work is required for an IOS router upgrade, configuration and fibre changes. BT Wholesale will endeavour to complete the work promptly.
Note that those subscribed to the RSS feed will also receive direct notification of the work.