Just seen the following on Twitter:
Cavendish Street completely cut off siege situation in progress.
Bloke gone berserk above Subway in Cavendish St, Chesterfield. Chucked a load of stuff into the road. Not nice, has a knife.
Incident on Cavendish Street drawing to a close, poor chap safely detained, we will be open in a few minutes.
Will see if I can find out more.