It was NAS that 'failed' not Swanwick. NAS suffered 3 startovers in rapid succession and to protect BOTH systems the link to Swanwick is dropped.
When Swanwick is operating in manual mode (without NAS) you MUST impose a zero flow rate until such time as you've sorted out the traffic and got some idea of exactly what the situation is. Its not a terribly good idea from a safety and an ATC viewpoint to allow flights to continue to enter your airspace when your capacity has been severely cut.
AFAIK since it has opened the Swanwick System has only 'failed' once, a problem with bandboxing and splitting. Every other purported failure of Swanwick has been down to NAS.
BD