Coats Milward Brand, fastener, that hooks on to the existing fastener to extend it, suits28mm wide fastener 

extends bra up to 14cm (5 1/2") (3 eyes to adjust)with 9cm (3 1/2") of elastic which can be shortened and the hook side them sewn on to attach and make removable, or you use to repair an existing bra with a damaged fastener