Who is responsible for existing flood defences?

According to DCLG (who superseded the ODPM):

Defra has overall policy responsibility for flood and coastal erosion risk in England and the Environment Agency has statutory responsibility for flood management and defence.

The Environment Agency, Internal Drainage Boards and local authorities are responsible for maintaining, operating and improving flood defences.

For further information please see

Planning for flood risk: the facts - Communities and Local Government