how can I fix brake lights which keep flickering on, on my Citroen Picasso?

Citroen Picasso,1.8 SX, X reg. Brake lights keep flickering on. Had this looked at by the Citroen Service centre who adjusted the switch according to their notes but problem persists. I believe that the pedal has "relaxed" and is sitting too low thus causing them to be oversensitive and flickering on when going over slightest bump in the road. Can the brake pedal be adjusted ? Is it a spring that might be worn ?
Anyone any ideas before Citroen want to charge small fortune to change switch ?