Cogent are in the process of performing some network upgrades. This appears to be having a larger impact than they have advertised and so is affecting any ISP who are routing traffic via them:
Cogent will be performing code upgrades in the following areas. During these upgrades, customers in or transiting the area may experience intermittent periods of packet loss and latency between 15 and 45 minutes for the duration of the window.
Location: London, UK
Start time: 1/6 19:00 ET
End time: 1/7 02:00 ET
Work order number: NC822-60
Location: London, UK; Southport, UK
Start time: 1/6 19:00 ET
End time: 1/7 02:00 ET
Work order number: NC822-58
This maintenance window is due to complete shortly and so hopefully the issue will then resolve.