Haberdashers just off Oxford Street. Sells things you never knew existed.
Basement contains Dylon dyes, possibly other fabric treatments, I can't remember off hand. Eyelets and popper snaps and tools for setting them. Upstairs - huge range of buttons, braid, lace, bag fittings etc. They also sell the bits for corset making. Shop is packed with stuff, but you need to ring the buzzer to be let in.